Leave boxing action, regularly missing jab (9)
I believe the answer is:
vaccinate
'jab' is the definition.
(I know that jab can be written as vaccinate)
'leave boxing action regularly missing' is the wordplay.
'leave' becomes 'vacate' (to vacate a place is to leave it).
'boxing' means one lot of letters goes inside another ('box' can mean contain, as in to put in a box).
'regularly missing' means one should take alternating letters (letters removed at regular intervals).
The alternating letters of 'action' are 'cin'.
'vacate' enclosing 'cin' is 'VACCINATE'.
(Other definitions for vaccinate that I've seen before include "Immunise" , "prevent disease" , "Inoculate with ..." .)