Posh chap accepts a sovereign for going (6,3)
I believe the answer is:
taking off
'going' is the definition.
(taking off is a kind of going)
'posh chap accepts a sovereign' is the wordplay.
'posh chap' becomes 'toff' (I've seen this before).
'accepts' means one lot of letters goes inside another.
'sovereign' becomes 'king' (king is a kind of sovereign).
'a'+'king'='aking'
'toff' going around 'aking' is 'TAKING OFF'.
'for' is the link.
(Other definitions for taking off that I've seen before include "Removing (clothes)" , "leaving the ground" .)