Become weak and die away through lack of use (7)
I believe the answer is:
atrophy
'become weak and die away through lack of use' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for atrophy that I've seen before include "shrivel" , "Waste away, wither" , "Waste away - sounds like a sporting award" , "Decline" , "Waste away as through lack of food" .)