Leguminous seed (6)
(Other definitions for lentil that I've seen before include "High-protein pulse" , "Vegetarian food" , "Edible leguminous plant seed" , "Pulse for soup etc" , "'Seed, pulse (6)'" .)
'leguminous seed' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pea that I've seen before include "Ape this plant badly" , "Tiny stone" , "Leguminous seed" , "Edible seed from pod" , "Climbing plant; vegetable" .)