When one's supporting the job centre, it's a haven (5)
I believe the answer is:
oasis
'a haven' is the definition.
('oasis' can be a synonym of 'haven')
'when one's supporting the job centre' is the wordplay.
'when' becomes 'as'.
'one's' becomes 'is' (roman numeral).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'centre' says to take the centre.
The middle of 'job' is 'o'.
'as'+'is'='asis'
'asis' after 'o' is 'OASIS'.
'it's' acts as a link.
(Other definitions for oasis that I've seen before include "Britpop group" , "Peaceful area away from trouble" , "Fertile place in a desert, watering-hole" , "Place of rest in the midst of gloom" , "Q-and-A game" .)