Colossal Scotsman in sports car (5)
I believe the answer is:
giant
'colossal' is the definition.
(I know that colossal can be written as giant)
'scotsman in sports car' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'in' indicates putting letters inside.
'sports car' becomes 'GT' (abbreviation for grand tourer).
'ian' going inside 'gt' is 'GIANT'.
(Other definitions for giant that I've seen before include "Gargantuan" , "Large mythical figure" , "Designating big" , "Superhuman being" , "Huge; James Dean film" .)