Cooking in 'lite' was something a model needed to watch, perhaps (9)
I believe the answer is:
waistline
'something a model needed to watch perhaps' is the definition.
'cooking in lite was' is the wordplay.
'cooking' indicates an anagram.
'in'+'lite'+'was'='inlitewas'
'inlitewas' is an anagram of 'WAISTLINE'.
(Other definitions for waistline that I've seen before include "Swine tail (anag.)" , "It goes round trunk" , "Girth" , "measurement" .)