Thinking nothing having drunk gin and tonic outside (9)
I believe the answer is:
cognition
'thinking' is the definition.
(I've seen this before)
'nothing having drunk gin and tonic outside' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'having drunk' indicates anagramming the letters (I've seen 'have drunk' mean this (I've seen this in other clues)).
'and' means one lot of letters go next to another.
'outside' means one lot of letters goes inside another.
'gin' put after 'tonic' is 'tonicgin'.
'tonicgin' with letters rearranged gives 'cognitin'.
'o' placed into 'cognitin' is 'COGNITION'.
(Other definitions for cognition that I've seen before include "The process of acquiring knowledge" , "gaining insight" , "Faculty of knowing while incognito" , "Knowledge - no to icing (anag)" , "Thinking" .)