What perhaps causes ice to melt (4)

I believe the answer is:

thaw

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'to melt' is the definition.
('thaw' can be a synonym of 'melt')

'what perhaps causes ice' is the wordplay.
'what' anagrams to 'THAW'.
However, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for thaw that I've seen before include "(Of snow or ice) melt" , "Begin to melt" , "detente" , "Morse actor; melt" , "snow does eventually" .)

I've seen this clue in The Globe and Mail.

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