Vast expansion of G & T — one tucks in (5)
I believe the answer is:
giant
'vast' is the definition.
(I know that vast can be written as giant)
'expansion of g t one tucks in' is the wordplay.
'expansion of g' becomes 'gin' (abbreviation as in G and T, gin and tonic).
'one' becomes 'a' (a thing is one thing).
'tucks in' is an insertion indicator.
'gin'+'t'='gint'
'gint' going around 'a' is 'GIANT'.
(Other definitions for giant that I've seen before include "Ogre" , "Huge; James Dean film" , "James Dean film (big one!)" , "Huge person" , "Creature described in 14 17" .)