London police, endless subject (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'london police endless' is the wordplay.
'london' becomes 'the' (I can't justify this - if you can you should give a lot more credence to this answer).
'police' becomes 'met' (London Metropolitan Police).
'endless' means to remove the last letter.
'met' with its final letter taken off is 'me'.
'the'+'me'='THEME'
Can you help me to learn more?
(Other definitions for theme that I've seen before include "Main idea or topic" , "A subject or repeated melody" , "Motif, topic" , "Subject (of talk, text, etc)" , "(Writer's) topic" .)