Tree is more comfortable, less cold (5)
I believe the answer is:
osier
'tree' is the definition.
(osier is a kind of tree)
'more comfortable less cold' is the wordplay.
'more comfortable' becomes 'cosier' ('cosy' can be similar in meaning to 'comfortable').
'less' indicates named letters should be taken away.
'cold' becomes 'c' (eg on taps).
'cosier' with 'c' removed is 'OSIER'.
'is' acts as a link.
(Other definitions for osier that I've seen before include "Wetland willow" , "Weaving cane for Rosie" , "Basket maker" , "Bit of tree" , "Basket-maker's willow" .)