Help police incursion when leader leaves (3)
I believe the answer is:
aid
'help police' is the definition.
'aid' can be an answer for 'help' (synonyms). I'm not certain of the 'police' bit.
'incursion when leader leaves' is the wordplay.
'incursion' becomes 'raid' (raid is a kind of incursion).
'when leader leaves' means to remove the first letter.
'raid' with its first letter removed is 'AID'.
Can you help me to learn more?
(Other definitions for aid that I've seen before include "Help or support" , "Help, assistance" , "A hand" , "Care, assistance" , "charity" .)