Metal brackets in case for instrument (4)
I believe the answer is:
fife
'for instrument' is the definition.
(fife is a kind of instrument)
'metal brackets in case' is the wordplay.
'metal' becomes 'fe' (Fe is an example).
'brackets' is an insertion indicator (I've seen this in other clues).
'in case' becomes 'if' (if means in a particular case).
'fe' enclosing 'if' is 'FIFE'.
(Other definitions for fife that I've seen before include "Small flute - ancient kingdom of central Scotland" , "St Andrews area" , "Scottish division" , "part of Scotland?" , "Musical instrument -- area of Scotland" .)