He has a piano for collection (4)
I believe the answer is:
heap
'for collection' is the definition.
(heap is a kind of collection)
'he has a piano' is the wordplay.
'has' means one lot of letters go next to another.
'piano' becomes 'p' (musical abbreviation).
'he'+'a'+'p'='HEAP'
(Other definitions for heap that I've seen before include "Pile, stack" , "Mound, pile" , "Mound, stack" , "Loose pile" , "good deal" .)