Onset of melting (4)
I believe the answer is:
thaw
'onset of melting' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for thaw that I've seen before include "Become more friendly" , "John, once regularly on TV" , "following 28 [SILVER]" , "Become less stiff, more relaxed" , "(Of snow or ice) melt" .)