Fast old tempo (5)
I believe the answer is:
lento
'tempo' is the definition.
(type of musical tempo)
'fast old' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lent'+'o'='LENTO'
(Other definitions for lento that I've seen before include "Instruction to players" , "It's slower than adagio" , "Slow musically" , "(Of tempo) slow" , "Slow (mus.)" .)