Introduction for piano arranged by unknown composer (5)
I believe the answer is:
parry
'composer' is the definition.
(Hubert Parry, 1848-1918)
'introduction for piano arranged by unknown' is the wordplay.
'introduction for' suggests taking the first letters.
'arranged' becomes 'arr' (abbreviation).
'by' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
The initial letter of 'piano' is 'p'.
'p'+'arr'+'y'='PARRY'
(Other definitions for parry that I've seen before include "Impede movement" , "Fend off (a blow)" , "Block or counterpunch" , "skilful evasion" , "Duck" .)