Perhaps Martin's partner in sports car is hard to shock (8)
I believe the answer is:
astonish
'shock' is the definition.
(thesaurus)
'perhaps martin's partner in sports car is hard' is the wordplay.
'perhaps martin's partner in sports car' becomes 'aston' (Aston Martin is a type of sports car).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'aston'+'is'+'h'='ASTONISH'
'to' acts as a link.
(Other definitions for astonish that I've seen before include "Greatly impress" , "Confound" , "'knock for six'?" , "Stagger" , "Gobsmack" .)