Man with son on Welsh river (5)
I believe the answer is:
staff
'man' is the definition.
(I've seen this before)
'son on welsh river' is the wordplay.
'son' becomes 's'.
'on' means one lot of letters go next to another.
'welsh river' becomes 'taff' (I've seen this before).
's'+'taff'='STAFF'
'with' acts as a link.
(Other definitions for staff that I've seen before include "crook" , "Rules needed for scoring" , "Employees; sceptre" , "See 11" , "Hands" .)