Special Air Service by hotel window (4)
I believe the answer is:
sash
'window' is the definition.
(I've seen this before)
'special air service by hotel' is the wordplay.
'special air service' becomes 'sas'.
'by' says to put 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 "Band worn over shoulder or round waist" , "(Uniform) strip; window-frame" , "Baldrick" , "Cummerbund" , "Strip of cloth worn round the waist" .)