Deputy's one of four in this part of the police force (4,5)
I believe the answer is:
vice squad
'the police force' is the definition.
(vice squad is a kind of police force)
'deputy's one of four' is the wordplay.
'deputy's' becomes 'vice's' (as in vice-president).
'one of four' becomes 'quad' (short for quadruplet, one of four children).
'vices'+'quad'='VICE SQUAD'
'in this part of' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for vice squad that I've seen before include "police department" , "Police unit enforcing gaming and prostitution laws" , "They tackle corruption" , "section of police force" .)