Trend to lose freshness mostly (3)
I believe the answer is:
fad
'trend' is the definition.
(a fad is a brief craze or fashion)
'lose freshness mostly' is the wordplay.
'lose freshness' becomes 'fade' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'fade' with its final letter taken off is 'FAD'.
'to' acts as a link.
(Other definitions for fad that I've seen before include "A passing liking or fashion" , "Temporary vogue or craze" , "A craze or current vogue" , "A passing fancy or fashion" , "transient success" .)