Group of actors in rep, out swimming (6)
I believe the answer is:
troupe
'group of actors' is the definition.
(I've seen this before)
'rep out swimming' is the wordplay.
'swimming' is an anagram indicator.
'rep'+'out'='repout'
'repout' is an anagram of 'TROUPE'.
'in' is the link.
(Other definitions for troupe that I've seen before include "Travelling company of actors" , "tour?" , "Performing company" , "Groupe of entertainers" , "Group of touring actors" .)