Leave for outer space with posh chap (3,3)
I believe the answer is:
set off
'leave' is the definition.
(set off can mean to depart)
'outer space with posh chap' is the wordplay.
'outer' suggests removing the centre.
'with' is a charade indicator (letters next to each other).
'posh chap' becomes 'toff' (I've seen this before).
'space' with its middle taken out is 'se'.
'se'+'toff'='SET-OFF'
'for' acts as a link.
(Other definitions for set off that I've seen before include "Begin journey or cause to ignite" , "show to advantage" , "Leave - explode" , "Commence journey or explode bomb" , "radio with no reception" .)