Old American gets hold of piano work (4)
I believe the answer is:
opus
'work' is the definition.
(I've seen this before)
'old american gets hold of piano' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us'.
'gets hold of' indicates putting letters inside.
'piano' becomes 'p'.
'o'+'us'='ous'
'ous' placed around 'p' is 'OPUS'.
(Other definitions for opus that I've seen before include "Artistic work" , "Work (mus.)" , "Composer's work" , "Composer's catalogue number" , "Major work" .)