Sap from lettuce hearts (4)
I believe the answer is:
cosh
'sap' is the definition.
('cosh' can be a synonym of 'sap')
'lettuce hearts' is the wordplay.
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'cos'+'h'='COSH'
'from' acts as a link.
(Other definitions for cosh that I've seen before include "Club used as a weapon" , "Blackjack" , "Blunt weapon, often of hard rubber" , "(Hit on the head with) a blunt weapon" , "Thick heavy stick as a weapon" .)