Justifying previous intention to drink one gin cocktail (10)
I believe the answer is:
explaining
'justifying' is the definition.
(explaining is a kind of justifying)
'previous intention to drink one gin cocktail' is the wordplay.
'previous' becomes 'ex' (I've seen this in other clues).
'intention' becomes 'plan'.
'to drink' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'cocktail' is an anagram indicator.
'plan' enclosing 'i' is 'plain'.
'gin' is an anagram of 'ing'.
'ex'+'plain'+'ing'='EXPLAINING'
(Other definitions for explaining that I've seen before include "Justifying" , "Describing" .)