What managed to defrost? (4)
I believe the answer is:
thaw
'defrost?' is the definition.
(defrosting is a kind of thawing)
'what managed' is the wordplay.
'managed' indicates anagramming the letters.
'what' with letters rearranged gives 'THAW'.
'to' is the link.
(Other definitions for thaw that I've seen before include "What - melt like snow?" , "Morse actor; melt" , "following 28 [SILVER]" , "Become more friendly" , "Frost going" .)