Well-trained police meet requirements (3,3,4)
I believe the answer is:
fit the bill
'requirements' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'well-trained police meet' is the wordplay.
'well-trained' becomes 'fit' (I've seen this in another clue).
'police' becomes 'the bill' (informal UK term for the police).
'meet' says to put letters next to each other.
'fit'+'thebill'='FIT THE BILL'
Can you help me to learn more?
(Other definitions for fit the bill that I've seen before include "What police tailors might do" , "finish off some taxidermy?" , "serve well" , "Be suitable" .)