Beginning to feel stiff and cold (6)
I believe the answer is:
frigid
'cold' is the definition.
('frigid' can be similar in meaning to 'cold')
'beginning to feel stiff' is the wordplay.
'beginning to' indicates taking the first letters.
'stiff' becomes 'rigid' ('rigid' can be a synonym of 'stiff').
The first letter of 'feel' is 'f'.
'f'+'rigid'='FRIGID'
'and' acts as a link.
(Other definitions for frigid that I've seen before include "Very cold; formal in behaviour" , "Icy" , "Glacial" , "Very cold in manner" , "Lacking affection or warmth" .)