On leave, this writer’s becoming unruly (7)
I believe the answer is:
restive
'unruly' is the definition.
(thesaurus)
'on leave this writer's' is the wordplay.
'on' is a charade indicator (letters next to each other).
'leave' becomes 'rest' (leave is a rest from work).
'this writer's' becomes 'ive' ('I have' can refer to the person writing).
'rest' next to 'ive' is 'RESTIVE'.
'becoming' acts as a link.
(Other definitions for restive that I've seen before include "Impatient of authority" , "It veers around, resisting control" , "Unable to stay still" , "Uneasy, nervous" , "Fidgety, uneasy" .)