Plays a part while holding court (4)
I believe the answer is:
acts
'plays a part' is the definition.
(I've seen this in another clue)
'while holding court' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'holding' means one lot of letters goes inside another.
'court' becomes 'ct' (abbreviation used in road names).
'as' placed around 'ct' is 'ACTS'.
(Other definitions for acts that I've seen before include "Theatrical deeds" , "Does something" , "Legal documents" , "exerts influence" , "functions" .)