Bolt's key skill (4)
I believe the answer is:
dart
'bolt's' is the definition.
(thesaurus)
'key skill' is the wordplay.
'key' becomes 'd' (I've seen this in other clues**).
'skill' becomes 'art' (I've seen this before).
'd'+'art'='DART'
(Other definitions for dart that I've seen before include "Small arrow" , "Move quickly and suddenly" , "Tuck; arrow" , "How a musketeer begins" , "One fired" .)