Instrument in boat capsized under fire (7)
I believe the answer is:
sackbut
'instrument' is the definition.
(sackbut is a kind of instrument)
'boat capsized under fire' is the wordplay.
'boat' becomes 'tub' (I've seen this before).
'capsized' shows that the letters should be reversed in order.
'under' is a charade indicator (letters next to each other).
'fire' becomes 'sack' (dismiss someone from their job).
'tub' written backwards gives 'but'.
'but' after 'sack' is 'SACKBUT'.
'in' is the link.
(Other definitions for sackbut that I've seen before include "Old trombone" , "Musical instrument of the Renaissance" , "music maker" , "Ancient musical instrument" , "one blows into it" .)