Big Scotsman stopping GT (5)
I believe the answer is:
giant
'big' is the definition.
(similar in meaning)
'scotsman stopping gt' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'stopping' is an insertion indicator (stopping can mean blocking a gap).
'ian' going inside 'gt' is 'GIANT'.
(Other definitions for giant that I've seen before include "Huge; James Dean film" , "Colossus" , "Designating big" , "Large black and white mammal" , "James Dean film (big one!)" .)