Sweetheart not afraid to swallow head of lettuce (7)
I believe the answer is:
darling
'sweetheart' is the definition.
(both are terms of address to romantic partner)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for darling that I've seen before include "Wendy's family (Peter Pan)" , "family in play" , "Grace -, nineteenth century wreck heroine of lighthouse" , "Victorian heroine" , "Australian river and loved one" .)