Instrument unassuming bandsmen initially used in army (4)
'instrument' is the definition.
(tuba is a kind of instrument)
'unassuming bandsmen initially used in army' is the wordplay.
'initially' indicates taking the first letters.
'used in' is an insertion indicator.
'army' becomes 'ta' (abbreviation for Territorial Army).
The first letters of 'unassuming bandsmen' is 'ub'.
'ub' inserted within 'ta' is 'TUBA'.
(Other definitions for tuba that I've seen before include "Lowest brass wind instrument", "Musical instrument -- 5 (rev)", "member of band", "Valved instrument", "Brass "oompah" instrument".)