Unfreezes (5)
I believe the answer is:
thaws
'unfreezes' is the definition.
('thaw' can be a synonym of 'unfreeze')
This is the entire clue.
(Other definitions for thaws that I've seen before include "Starts to unfreeze" , "(Of snow) melts" , "As role of Morse was to become" , "friendlier times?" .)