Unit of the enemy housed in HQ (10)
I believe the answer is:
centimetre
'unit' is the definition.
(centimetre is a kind of unit)
'enemy housed in hq' is the wordplay.
'enemy' becomes 'time' (proverbially, 'time is the enemy').
'housed in' is an insertion indicator.
'hq' becomes 'centre' (centre can mean a headquarters or base).
'time' placed within 'centre' is 'CENTIMETRE'.
'of the' is the link.
(Other definitions for centimetre that I've seen before include "From a little distance" , "Measure" , "not giving an inch then?" , "About four tenths of an inch" , "Hundredth of a metre" .)