Deteriorating swimming resort's losing special quality (10)
I believe the answer is:
retrograde
'deteriorating' is the definition.
(synonyms)
'swimming resort's losing special quality' is the wordplay.
'swimming' indicates an anagram.
'losing' is a deletion indicator.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'quality' becomes 'grade' (I've seen this before).
'resort' with 's' taken away is 'reort'.
'reort' with letters rearranged gives 'retro'.
'retro'+'grade'='RETROGRADE'
(Other definitions for retrograde that I've seen before include "turning back" , "Reverse" , "Reactionary" , "Declining" , "Going back" .)