Capital billets for operations base (12)
I believe the answer is:
headquarters
'operations base' is the definition.
'headquarters' can be an answer for 'base' (I've seen this before). I'm not sure about the 'operations' bit.
'capital billets' is the wordplay.
'capital' becomes 'head'.
'billets' becomes 'quarters' ('quarter' can be a synonym of 'billet').
'head'+'quarters'='HEADQUARTERS'
'for' is the link.
Can you help me to learn more?
(Other definitions for headquarters that I've seen before include "Centre of operations" , "Administrative centre" , "Command centre" , "Military command post" , "Operational centre" .)