Belt commandos hard (4)
I believe the answer is:
sash
'belt' is the definition.
(thesaurus)
'commandos hard' is the wordplay.
'commandos' becomes 'sas' (Special Air Service).
'hard' becomes 'h'.
'sas'+'h'='SASH'
(Other definitions for sash that I've seen before include "Girdle" , "Baldrick" , "Waistband - type of window" , "Decorative band" , "Decoration for the twelfth" .)