Massive Scotsman into GT (5)
I believe the answer is:
giant
'massive' is the definition.
(synonyms)
'scotsman into gt' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'into' means one lot of letters goes inside another.
'ian' placed inside 'gt' is 'GIANT'.
(Other definitions for giant that I've seen before include "Monster" , "Titan" , "Large black and white mammal" , "James Dean film (big one!)" , "Creature of exceptional size" .)