Cold, and beginning to feel stiff (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' suggests 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 "Lacking affection or warmth" , "Very cold in manner" , "Very cold; formal in behaviour" , "Icy" , "Sexually unresponsive" .)