Man swimming in a loch beginning to shiver (8)
I believe the answer is:
nicholas
'man' is the definition.
'swimming in a loch beginning to shiver' is the wordplay.
'swimming' indicates an anagram.
'beginning to' suggests taking the first letters.
The first letter of 'shiver' is 's'.
'in'+'a'+'loch'='inaloch'
'inaloch' anagrammed gives 'nichola'.
'nichola'+'s'='NICHOLAS'
(Other definitions for nicholas that I've seen before include "Patron saint of children and Russia" , "Boy" , "Saint identified as Santa Claus" , "Former Russian leader" , "Saint associated with Christmas figure" .)