Whopping G&T Scot tucks into (5)
I believe the answer is:
giant
'whopping' is the definition.
(both can mean very big)
'gt scot tucks into' is the wordplay.
'scot' becomes 'ian'.
'tucks into' means one lot of letters goes inside another.
'gt' placed around 'ian' is 'GIANT'.
(Other definitions for giant that I've seen before include "Titan" , "Huge; James Dean film" , "Very big chap" , "Very big person" , "Designating big" .)