Book when holding court (4)
I believe the answer is:
acts
'book' is the definition.
(I've seen this before)
'when holding court' is the wordplay.
'when' becomes 'as'.
'holding' is an insertion indicator.
'court' becomes 'ct'.
'as' going around 'ct' is 'ACTS'.
(Other definitions for acts that I've seen before include "Does something" , "5th NT book" , "Fifth NT book" , "placed before Romans?" , "Cast in exploits" .)