Diet may change when it's light (7)
I believe the answer is:
daytime
'when it's light' is the definition.
(I've seen this before)
'diet may change' is the wordplay.
'change' indicates anagramming the letters.
'diet'+'may'='dietmay'
'dietmay' with letters rearranged gives 'DAYTIME'.
(Other definitions for daytime that I've seen before include "May diet (anag.)" , "Hours of light" , "Sunrise to sunset" , "Period when light" , "Period between dawn and dusk" .)