Remarking upon a mixture of gin and tonic (8)
I believe the answer is:
noticing
'remarking upon' is the definition.
('notice' can be a synonym of 'remark upon')
'a mixture of gin and tonic' is the wordplay.
'a mixture of' indicates anagramming the letters (letters mixed up).
'and' says to put letters next to each other.
'gin' put after 'tonic' is 'tonicgin'.
'tonicgin' is an anagram of 'NOTICING'.
(Other definitions for noticing that I've seen before include "Spotting" , "Perceiving" , "Seeing, detecting" , "Becoming aware of" , "Remarking" .)