Fruit pulp always first class (6)
I believe the answer is:
papaya
'fruit' is the definition.
(I know that papaya is a type of fruit)
'pulp always first class' is the wordplay.
'pulp' becomes 'pap' (both can mean worthless or trivial stuff).
'always' becomes 'ay' (archaic term for 'always').
'first class' becomes 'A' (as in A-list, A-team etc).
'pap'+'ay'+'a'='PAPAYA'
(Other definitions for papaya that I've seen before include "Tropical fruit tree used in medicine" , "Caribbean fruit" , "Tropical green-skinned fruit" , "Oval melon-like tropical fruit" , "Tropical fruit with pink flesh and black seeds" .)