Begin to melt (4)
I believe the answer is:
thaw
'begin to melt' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for thaw that I've seen before include "Melt in heat" , "Become friendlier; melt" , "(Of snow or ice) melt" , "Frost going" , "Become liquid" .)