Period of working on hobby regularly a compulsion (9)
I believe the answer is:
obsession
'a compulsion' is the definition.
('obsession' can be a synonym of 'compulsion')
'period of working on hobby regularly' is the wordplay.
'period of working' becomes 'session'.
'on' is a charade indicator (letters next to each other).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'hobby' are 'ob'.
'session' put after 'ob' is 'OBSESSION'.
(Other definitions for obsession that I've seen before include "Compulsion, fixation" , "Continual preoccupation" , "Persistent preoccupation" , "Complex" , "Phobia" .)