Slow piece loaned by leader of orchestra (5)
I believe the answer is:
lento
'slow piece' is the definition.
(musical term)
'loaned by leader of orchestra' is the wordplay.
'loaned' becomes 'lent' ('lend' can be a synonym of 'loan').
'by' is a charade indicator (letters next to each other).
'leader of' indicates taking the first letters.
The initial letter of 'orchestra' is 'o'.
'lent'+'o'='LENTO'
(Other definitions for lento that I've seen before include "Slow (music)" , "(When playing music) slowly" , "Slow musically" , "It's slower than adagio" , "(Of tempo) slow" .)