Mentally assessed one final letter by editor during drink (5,2)
I believe the answer is:
sized up
'mentally assessed' is the definition.
Although both the answer and definition are past participle verbs, I don't see how one could define the other.
'one final letter by editor during drink' is the wordplay.
'one final letter' becomes 'zed' (in the alphabet).
'by editor' becomes 'u' (I can't explain this - if you can you should believe this answer much more).
'during' indicates putting letters inside.
'drink' becomes 'sip' (sipping is a kind of drinking).
'zed'+'u'='zedu'
'zedu' going inside 'sip' is 'SIZED UP'.
Can you help me to learn more?
(Other definitions for sized up that I've seen before include "Estimated (the) dimensions (of)" , "Used zip (anag.)" .)