What managed to defrost? (4)
I believe the answer is:
thaw
'to defrost?' is the definition.
(defrosting is a kind of thawing)
'what managed' is the wordplay.
'managed' indicates an anagram.
'what' is an anagram of 'THAW'.
(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" .)