When to live outside HQ (4)
I believe the answer is:
base
'hq' is the definition.
(I've seen this before)
'when to live outside' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'to live' becomes 'be' (synonyms).
'outside' means one lot of letters goes inside another.
'as' inserted within 'be' is 'BASE'.
(Other definitions for base that I've seen before include "Shameful - headquarters" , "Interest charged by the Bank of England to commercial banks" , "centre from which soldiers operate" , "This underlying structure" , "Low kind of foundation" .)