Agreement rejected after mother left drop of mulled wine (7)
I believe the answer is:
malmsey
'wine' is the definition.
(malmsey is a type of wine)
'agreement rejected after mother left drop of mulled' is the wordplay.
'agreement' becomes 'yes'.
'rejected' is a reversal indicator.
'after' says to put letters next to each other.
'mother left' becomes 'mal' (I can't explain this - if you can you should believe this answer much more).
'drop of' indicates taking the first letters.
The first letter of 'mulled' is 'm'.
'yes' written backwards gives 'sey'.
'mal'+'m'='malm'
'sey' after 'malm' is 'MALMSEY'.
(Other definitions for malmsey that I've seen before include "Sweet Madeira wine" , "Fortified Madeira wine" .)