To be played slowly, but in fast time by the orchestra leader (5)
I believe the answer is:
lento
'to be played slowly' is the definition.
(musical instruction)
'time by the orchestra leader' is the wordplay.
'time' becomes 'lent' (I have seen 'Time of fasting ' mean 'lent' so perhaps 'time' could also mean 'lent').
'by' says to put letters next to each other.
'leader' says to take the initial letters.
The first letter of 'orchestra' is 'o'.
'lent'+'o'='LENTO'
'but in fast' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lento that I've seen before include "Slow musically" , "Slow passage" , "Old sayings" , "How one scores slowly" , "It's slower than adagio" .)