Head of lettuce swallowed by bold sweetheart (7)
I believe the answer is:
darling
'sweetheart' is the definition.
(both are terms of address to romantic partner)
'head of lettuce swallowed by bold' is the wordplay.
'head of' suggests taking the first letters.
'swallowed by' means one lot of letters goes inside another.
'bold' becomes 'daring' (I've seen this before).
The initial letter of 'lettuce' is 'l'.
'l' put inside 'daring' is 'DARLING'.
(Other definitions for darling that I've seen before include "Adorable - honeybun" , "Film with Julie Christie and Dirk Bogarde - sweetheart" , "Term of affection" , "lover" , "Term of endearment" .)