Demand when king (3)
I believe the answer is:
ask
'demand' is the definition.
(I know that demand can be written as ask)
'when king' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'king' becomes 'K' (abbreviation for king in chess).
'as'+'k'='ASK'
(Other definitions for ask that I've seen before include "Inquire" , "Put a question" , "Invite (to a party)" , "Make enquiry" , "Request, invite" .)