Salvation Army save up for instruments (5)
I believe the answer is:
tubas
'for instruments' is the definition.
(I've seen this in another clue)
'salvation army save up' is the wordplay.
'salvation army' becomes 'SA' (abbreviation).
'save' becomes 'but' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'sa'+'but'='sabut'
'sabut' back-to-front is 'TUBAS'.
(Other definitions for tubas that I've seen before include "Large brass instruments" , "What some musicians" .)