Save up to get a brass instrument (4)
I believe the answer is:
tuba
'brass instrument' is the definition.
(I've seen this before)
'save up to get a' is the wordplay.
'save' becomes 'but' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to get' means one lot of letters go next to another.
'but' in reverse letter order is 'tub'.
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "part of 14 [ORCHESTRA] ?" , "item band might play" , "Stop" , "This loud instrument" , "blow it!" .)