Under fire, however, players blew it (7)
I believe the answer is:
Here is my best explanation:
'players blew it' is the definition.
(historical type of trombone)
'under fire however' is the wordplay.
'under' says to put letters next to each other.
'fire' becomes 'sack' (dismiss someone from their job).
'however' becomes 'but' (I've seen this before).
'sack'+'but' is 'SACKBUT'.
(Other definitions for sackbut that I've seen before include "Early form of trombone", "Old trombone", "music-maker", "Medieval form of trombone", "Ancient musical instrument", "music maker", "one on the slide", "Old instrument", "Instrument" and "one blows into it".)