With one against playing back slow piece of music (9)
I believe the answer is:
andantino
'slow piece of music' is the definition.
(term in classical music)
'with one against playing back' is the wordplay.
'with' becomes 'and'.
'one against' becomes 'anti' (I've seen this before).
'playing' becomes 'on'.
'back' shows that the letters should be reversed in order.
'on' in reverse letter order is 'no'.
'and'+'anti'+'no'='ANDANTINO'
(Other definitions for andantino that I've seen before include "Musical tempo - ain't donna (anag)" , "Slightly faster than walking pace (mus.)" , "At a light moderate pace (music)" , "a slowish pace" , "Little faster than moderately slow (music)" .)