Suitable form of high-speed transport (3)
I believe the answer is:
apt
I believe this clue is a double definition.
'suitable' is the first definition.
(thesaurus)
'form of high-speed transport' is the second definition.
(abbreviation for advanced passenger train)
(Other definitions for apt that I've seen before include "clever" , "may be appropriate" , "Befitting" , "Suitable or likely" , "Suitable; quick to learn" .)