Average cost of what's fashionable? (8)
I believe the answer is:
moderate
'what's fashionable?' is the definition.
I can't tell whether this defines the answer.
'average cost' is the wordplay.
'average' becomes 'mode' (mode is a kind of average).
'cost' becomes 'rate' (rate is a kind of cost).
'mode'+'rate'='MODERATE'
'of' acts as a link.
(Other definitions for moderate that I've seen before include "Within reasonable or average limits" , "kept within bounds" , "Not extreme - not very good" , "Average in amount" , "Average, middle rate" .)