Mushroom added with first of lentils (5)
I believe the answer is:
morel
'mushroom' is the definition.
(I know that morel is a type of mushroom)
'added with first of lentils' is the wordplay.
'added' becomes 'more' (similar in meaning).
'with' says to put letters next to each other.
'first of lentils' becomes 'l' (1st letter of 'lentils').
'more'+'l'='MOREL'
(Other definitions for morel that I've seen before include "Edible mushroom" , "Type of mushroom" , "One has a cap" , "Edible fungus" .)