Get on well with one posh bloke after bonk (3,2,3)
I believe the answer is:
hit it off
'get on well' is the definition.
'one posh bloke after bonk' is the wordplay.
'one' becomes 'i' (Roman numeral).
'posh bloke' becomes 'toff' (informal term for an upper class person).
'after' means one lot of letters go next to another.
'bonk' becomes 'hit' (bonk can mean to hit or strike).
'i'+'toff'='itoff'
'itoff' after 'hit' is 'HIT IT OFF'.
'with' is the link.
(Other definitions for hit it off that I've seen before include "Get on well - with a blow?" , "Become immediate friends" , "Agree - get on well together" , "Be well suited" , "Develop a good friendly relationship" .)