Sundry underwater swimmers (6)
I believe the answer is:
divers
This clue might be a double definition.
'sundry' is the first definition.
(I've seen this before)
'underwater swimmers' is the second definition.
'divers' can be an answer for 'swimmers' (diver is a kind of swimmer). I am unsure of the 'underwater' bit.
Can you help me to learn more?
(Other definitions for divers that I've seen before include "people taking the plunge" , "ducks" , "footballers going to the ground, needlessly" , "people going to bed?" , "competitors at the Olympic Games?" .)