Commandos on hotel window (4)
I believe the answer is:
sash
'window' is the definition.
(I've seen this before)
'commandos on hotel' is the wordplay.
'commandos' becomes 'sas' (Special Air Service).
'on' is a charade indicator (letters next to each other).
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'sas'+'h'='SASH'
(Other definitions for sash that I've seen before include "Popular band in Northern Ireland" , "Baldrick" , "Window frame; cloth strip (worn over shoulder)" , "Framework holding window panes" , "Decorative belt" .)