He started to write up agreement, then served up this spirit mixed into evening drink (8)
I believe the answer is:
nightcap
'evening drink' is the definition.
(I know that evening drink can be written as nightcap)
'he started to write up agreement' is the wordplay.
'he' becomes 'nigh' (I can't justify this - if you can you should give a lot more credence to this answer).
'started' is a charade indicator (letters next to each other).
'to write up' says the letters should be written in reverse.
'agreement' becomes 'pact' (a pact is an agreement or deal).
'pact' backwards is 'tcap'.
'nigh'+'tcap'='NIGHTCAP'
'then served up this spirit mixed into' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nightcap that I've seen before include "Head covering" , "Patching (anag.)" , "Take before bed or wear in bed" , "what last drink might be" , "something you get at the eleventh hour?" .)