Fast finish for concerto's slow movement (5)
I believe the answer is:
lento
'slow movement' is the definition.
(I've seen this in another clue)
'fast finish for concerto's' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'finish for' indicates one should take the final letters (the letter which finishes the word).
The final letter of 'concerto' is 'o'.
'lent'+'o'='LENTO'
(Other definitions for lento that I've seen before include "Old sayings" , "Slowly (mus.)" , "Instruction to players" , "Slow musically" , "Scored slowly" .)