Key worker in action (9)
I believe the answer is:
operative
This clue is a double definition.
'key worker' is the first definition.
(an operative is a worker)
'in action' is the second definition.
(in operation)
(Other definitions for operative that I've seen before include "Significant" , "Working (person)" , "American detective" , "Machinist - working" , "Having force - skilled worker" .)