River Tees - are French diving in? (4)
I believe the answer is:
test
'river' is the definition.
(the River Test)
'tees are french diving in?' is the wordplay.
I cannot quite see how this works, but
'are french' could be 'es' ('are' in French) and 'es' is found within the answer.
'tees' could be 'tt' (two T's or teas) and 'tt' is found in the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for test that I've seen before include "criterion" , "water flowing through Hampshire" , "Trial - big match" , "Exam to play international cricket" , "catch trout in it?" .)