Plays part when outside court (4)
I believe the answer is:
acts
'plays part' is the definition.
('act' is part of 'play')
'when outside court' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'outside' is an insertion indicator.
'court' becomes 'ct' (abbreviation used in road names).
'as' enclosing 'ct' is 'ACTS'.
(Other definitions for acts that I've seen before include "Cast down by deeds" , "book form" , "Theatrical deeds" , "Deeds; book of the Bible" , "functions" .)