Finally, Usain Bolt cramped (6)
I believe the answer is:
narrow
'cramped' is the definition.
(thesaurus)
'finally usain bolt' is the wordplay.
'finally' indicates one should take the final letters.
'bolt' becomes 'arrow' (I've seen this before).
The final letter of 'usain' is 'n'.
'n'+'arrow'='NARROW'
(Other definitions for narrow that I've seen before include "contract" , "Pinched" , "Limited in scope; not wide" , "Small in breadth" , "Vessel designed for canals" .)