Observing gin and tonic being drunk (8)
I believe the answer is:
noticing
'observing' is the definition.
(noticing is a kind of observing)
'gin and tonic being drunk' is the wordplay.
'and' means one lot of letters go next to another.
'being drunk' indicates anagramming the letters.
'gin' put after 'tonic' is 'tonicgin'.
'tonicgin' anagrammed gives 'NOTICING'.
(Other definitions for noticing that I've seen before include "Spotting" , "Remarking" , "Becoming aware of" , "Observing" , "Perceiving" .)