Number swimming in Tyne (6)
I believe the answer is:
ninety
'number' is the definition.
(ninety is a kind of number)
'swimming in tyne' is the wordplay.
'swimming' is an anagram indicator.
'in'+'tyne'='intyne'
'intyne' with letters rearranged gives 'NINETY'.
(Other definitions for ninety that I've seen before include "6 x 3 x 5" , "Minutes played in a football match" , "Fourscore and ten" , "''Oh, the craic was ... in the Isle of Man'' (The Dubliners)" , "Minutes in a soccer game" .)