Get on well, having success with one swell guy (3,2,3)
I believe the answer is:
hit it off
'get on well' is the definition.
'success with one swell guy' is the wordplay.
'success' becomes 'hit' (hit is a kind of success).
'with' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'swell guy' becomes 'toff' (swell can mean an upper-class person).
'hit'+'i'+'toff'='HIT IT OFF'
'having' is the link.
(Other definitions for hit it off that I've seen before include "Get on well with - or knock from position?" , "Get on well - with a blow?" , "Be well suited" , "Become compatible and friendly" , "Agree - get on well together" .)