Old man having one round for twosome? (7)
I believe the answer is:
pairing
'twosome?' is the definition.
(I've seen this before)
'old man having one round' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'having' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'round' becomes 'ring' ('ring' can be a synonym of 'round'**).
'pa'+'i'+'ring'='PAIRING'
'for' is the link.
(Other definitions for pairing that I've seen before include "parliamentary procedure" , "outcome of cup draw" , "Twosome" , "Couple" .)