Son never swimming in river (6)
I believe the answer is:
severn
'river' is the definition.
(I've seen this before)
'son never swimming in' is the wordplay.
'son' becomes 's' ().
'swimming' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'never' is an anagram of 'evern'.
's' inserted inside 'evern' is 'SEVERN'.
(Other definitions for severn that I've seen before include "Nerves (anag.)" , "Longest river in Britain" , "Flower" , "Gloucester's river" , "River flowing into the Bristol Channel" .)