Start to chew huge lettuce (3)
I believe the answer is:
cos
'lettuce' is the definition.
(cos lettuce is a kind of lettuce)
'start to chew huge' is the wordplay.
'start to' indicates taking the first letters.
'huge' becomes 'os' (abbreviation for outsize).
The initial letter of 'chew' is 'c'.
'c'+'os'='COS'
(Other definitions for cos that I've seen before include "Salad ingredient" , "Island in Mediterranean" , "often associated with sin" , "vegetable?" , "Variety of lettuce" .)