Twitches after work in study (6)
I believe the answer is:
optics
'study' is the definition.
(optics is a kind of study)
'twitches after work' is the wordplay.
'twitches' becomes 'tics' (tic is a kind of twitch).
'after' is a charade indicator (letters next to each other).
'work' becomes 'op' (abbreviation for opus).
'tics' put after 'op' is 'OPTICS'.
'in' acts as a link.
(Other definitions for optics that I've seen before include "The scientific study of light and vision" , "Drink measuring devices" , "Spirit measures" , "Study of the physical properties of light" , "Science concerned with vision" .)