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' means to remove the middle letters.
'with' says to put 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" .)