Parent drinks a gin, disposing of one big bottle (6)
I believe the answer is:
magnum
'big bottle' is the definition.
(I've seen this in another clue)
'parent drinks a gin disposing of one' is the wordplay.
'parent' becomes 'mum' (mum is a kind of parent).
'drinks' indicates putting letters inside.
'disposing of' suggests deleting specific letters.
'one' becomes 'I' (Roman numeral).
'a'+'gin'='agin'
'agin' with 'i' taken away is 'agn'.
'mum' enclosing 'agn' is 'MAGNUM'.
(Other definitions for magnum that I've seen before include "Double-size wine bottle" , "Two wine bottles in one" , "Large wine bottle" , "that may be full of slugs" , "Wine bottle twice normal size" .)