Mountie swimming on a lake, becoming cold (11)
I believe the answer is:
unemotional
'cold' is the definition.
(thesaurus)
'mountie swimming on a lake' is the wordplay.
'swimming' is an anagram indicator.
'lake' becomes 'l'.
'mountie' is an anagram of 'unemoti'.
'unemoti'+'on'+'a'+'l'='UNEMOTIONAL'
'becoming' acts as a link.
(Other definitions for unemotional that I've seen before include "Cold" , "Without personal feelings" .)