Gargantuan Scotsman stuffed into sports car (5)
I believe the answer is:
giant
'gargantuan' is the definition.
('giant' can be a synonym of 'gargantuan')
'scotsman stuffed into sports car' is the wordplay.
'scotsman' becomes 'ian'.
'stuffed into' is an insertion indicator.
'sports car' becomes 'gt' (abbreviation for grand tourer).
'ian' placed into 'gt' is 'GIANT'.
(Other definitions for giant that I've seen before include "Extremely large (person)" , "Very big person" , "Large black and white mammal" , "Creature of exceptional size" , "Titan" .)