Act as glutton, devouring ends of lettuce messily (11)
I believe the answer is:
overindulge
'act as glutton' is the definition.
(I know this)
'devouring ends of lettuce messily' is the wordplay.
'ends of' says to hollow out the word (remove centre letters).
'messily' indicates an anagram.
'lettuce' with its centre taken out is 'le'.
'devouring'+'le'='devouringle'
'devouringle' anagrammed gives 'OVERINDULGE'.
(Other definitions for overindulge that I've seen before include "Pander to excessively" , "pig" , "Take to excess" , "Spoil through lavish treatment" , "Drink excessively" .)