Cause expert politician to be on time (6)
I believe the answer is:
prompt
'cause' is the definition.
(I know that prompt is a more specific form of the action cause)
'expert politician to be on time' is the wordplay.
'expert' becomes 'pro' (short for professional).
'politician' becomes 'MP' (Member of Parliament).
'to be on' says to put letters next to each other (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'pro'+'mp'+'t'='PROMPT'
(Other definitions for prompt that I've seen before include "Bring about; on time" , "Spurs do this" , "Punctual or timely" , "Whisper line to forgetful actor" , "Bring about; done without delay" .)