It's hot among swimming teams in river (6)
I believe the answer is:
thames
'river' is the definition.
(I've seen this before)
'it's hot among swimming teams' is the wordplay.
'it's hot' becomes 'h'.
'among' means one lot of letters goes inside another.
'swimming' indicates an anagram.
'teams' with letters rearranged gives 'tames'.
'h' placed inside 'tames' is 'THAMES'.
'in' acts as a link.
(Other definitions for thames that I've seen before include "The Ma's in London" , "'Old Father ___'" , "London flood control" , "Sam the river" , "London's river (6)" .)