Burn most of map (4)
I believe the answer is:
char
'burn' is the definition.
(charring is a kind of burning)
'most of map' is the wordplay.
'most of' means to remove the last letter.
'map' becomes 'chart' (chart is a kind of map).
'chart' with its last letter removed is 'CHAR'.
(Other definitions for char that I've seen before include "Make black by burning, say toast" , "Blacken by burning" , "Partially burn; fish" , "Scorch - fish - cleaning lady" , "toast far too much?" .)