Frenchman let out to defrost (4)
I believe the answer is:
melt
'to defrost' is the definition.
(I've seen this in another clue)
'frenchman let out' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur).
'out' indicates an anagram (out can mean wrong or inaccurate).
'let' is an anagram of 'elt'.
'm'+'elt'='MELT'
(Other definitions for melt that I've seen before include "Unfreeze" , "Become liquid by heating" , "(Of snow) turn to water" , "Turn to liquid" , "Calm" .)