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