Expert collects hard pine (4)
I believe the answer is:
ache
'pine' is the definition.
(I've seen this before)
'expert collects hard' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'collects' means one lot of letters goes inside another.
'hard' becomes 'h'.
'ace' enclosing 'h' is 'ACHE'.
(Other definitions for ache that I've seen before include "strong feeling" , "agony" , "Throb - hurt" , "Dull prolonged pain" , "Play up" .)