Pouches in which commandos keep cape (4)
I believe the answer is:
sacs
'pouches' is the definition.
(I know this)
'commandos keep cape' is the wordplay.
'commandos' becomes 'SAS' (Special Air Service).
'keep' means one lot of letters goes inside another.
'cape' becomes 'c' (abbreviation used on maps).
'sas' going around 'c' is 'SACS'.
'in which' is the link.
(Other definitions for sacs that I've seen before include "bags" , "Pouch-like parts of the body" .)