Elite troops last seen in oompah band (4)
I believe the answer is:
sash
'band' is the definition.
(a band of material)
'elite troops last seen in oompah' is the wordplay.
'elite troops' becomes 'SAS' (Special Air Service).
'last seen in' suggests the final letters.
The final letter of 'oompah' is 'h'.
'sas'+'h'='SASH'
(Other definitions for sash that I've seen before include "Popular band in Northern Ireland" , "Decorative band" , "Cummerbund" , "Type of sliding window" , "Strip of cloth - type of window" .)