Special Air Service at hotel window (4)
I believe the answer is:
sash
'window' is the definition.
(I've seen this before)
'special air service at hotel' is the wordplay.
'special air service' becomes 'sas'.
'at' means one lot of letters go next to another.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'sas'+'h'='SASH'
(Other definitions for sash that I've seen before include "Window frame; silk band" , "Style of window" , "Girdle" , "Ribbon as a symbol of rank" , "Type of sliding window" .)