Elite soldiers bashing in middle window-frame (4)
I believe the answer is:
sash
'window-frame' is the definition.
(I've seen this before)
'elite soldiers bashing in middle' is the wordplay.
'elite soldiers' becomes 'sas' (Special Air Service).
'in middle' means to look at the middle letters.
The middle of 'bashing' is 'h'.
'sas'+'h'='SASH'
(Other definitions for sash that I've seen before include "Broad band, the one me father wore?" , "Waistband - type of window" , "Brown bread for waist or shoulder" , "type of cord?" , "Strip of cloth worn round the waist" .)