Fraternise during swimming lessons (6)
I believe the answer is:
mingle
'fraternise' is the definition.
(thesaurus)
'during swimming lessons' is the wordplay.
'during' indicates a hidden word.
'MINGLE' is hidden within 'swimming lessons'.
(Other definitions for mingle that I've seen before include "Mix - associate" , "Circulate at a party" , "Mix together" , "Mix among others" , "Dawn song, poem" .)