One way to assign a weight to a variable is by changing its scale. The trick works for the clustering algorithms you mention, viz. k-means, weighted-average linkage and average-linkage.
Kaufman, Leonard, and Peter J. Rousseeuw. "Finding groups in data: An introduction to cluster analysis." (2005) - page 11:
The choice of measurement units gives rise to relative weights of the
variables. Expressing a variable in smaller units will lead to a
larger range for that variable, which will then have a large effect on
the resulting structure. On the other hand, by standardizing one
attempts to give all variables an equal weight, in the hope of
achieving objectivity. As such, it may be used by a practitioner who
possesses no prior knowledge. However, it may well be that some
variables are intrinsically more important than others in a particular
application, and then the assignment of weights should be based on
subject-matter knowledge (see, e.g., Abrahamowicz, 1985).
On the other hand, there have been attempts to devise clustering
techniques that are independent of the scale of the variables
(Friedman and Rubin, 1967). The proposal of Hardy and Rasson (1982) is
to search for a partition that minimizes the total volume of the
convex hulls of the clusters. In principle such a method is invariant
with respect to linear transformations of the data, but unfortunately
no algorithm exists for its implementation (except for an
approximation that is restricted to two dimensions). Therefore, the
dilemma of standardization appears unavoidable at present and the
programs described in this book leave the choice up to the user
Abrahamowicz, M. (1985), The use of non-numerical a pnon information for
measuring dissimilarities, paper presented at the Fourth European Meeting of
the Psychometric Society and the Classification Societies, 2-5 July, Cambridge
Friedman, H. P., and Rubin, J. (1967), On some invariant criteria for grouping data.
J . Amer. Statist. ASSOC6.,2 , 1159-1178.
Hardy, A., and Rasson, J. P. (1982), Une nouvelle approche des problemes de
classification automatique, Statist. Anal. Donnies, 7, 41-56.