Referring to centres for swimming, a CA quit, upset (7)
I believe the answer is:
aquatic
'referring to centres for swimming' is the definition.
(as in aquatic centres)
'a ca quit upset' is the wordplay.
'upset' is an anagram indicator.
'ca'+'quit'='caquit'
'caquit' anagrammed gives 'quatic'.
'a'+'quatic'='AQUATIC'
(Other definitions for aquatic that I've seen before include "Plant living in water" , "Growing in or near water" , "Growing or living in water" , "20 [TERRAPIN] is" , "Marine plant" .)