Boats carrying a number of instruments (5)
I believe the answer is:
tubas
'number of instruments' is the definition.
'boats carrying a' is the wordplay.
'boats' becomes 'tubs' (tub can informally mean a boat).
'carrying' indicates putting letters inside.
'tubs' going around 'a' is 'TUBAS'.
(Other definitions for tubas that I've seen before include "What some musicians" , "Large brass instruments" .)