Ask for melted cheese on top of burger (7)
I believe the answer is:
beseech
'ask' is the definition.
(I've seen this before)
'melted cheese on top of burger' is the wordplay.
'melted' is an anagram indicator.
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The initial letter of 'burger' is 'b'.
'cheese'+'b'='cheeseb'
'cheeseb' is an anagram of 'BESEECH'.
'for' is the link.
(Other definitions for beseech that I've seen before include "request, urgently" , "Entreat, implore" , "Beg earnestly, implore" , "wood" , "Request earnestly" .)