Icebreaker? (4)
I believe the answer is:
thaw
'icebreaker?' 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 "Begin to melt" , "(Of ice/snow) melt" , "Become liquid" , "John, once regularly on TV" , "get warmer" .)