Lightning strike brings in first of pickets (5)
I believe the answer is:
rapid
'lightning' is the definition.
(both can mean very fast)
'strike brings in first of pickets' is the wordplay.
'strike' becomes 'raid'.
'brings in' indicates putting letters inside.
'first of pickets' becomes 'p' (1st letter of 'pickets').
'raid' enclosing 'p' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Happening at great speed" , "Happening quickly" , "Showing pace" , "haste" , "Swift, speedy" .)