It keeps locks in good order (9)
I believe the answer is:
hairbrush
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'locks in good order' is the wordplay.
'locks' becomes 'hair' (locks can mean hair).
'in good order' becomes 'brush' (I can't explain this - if you can you should give a lot more credence to this answer).
'hair'+'brush'='HAIRBRUSH'
'keeps' acts as a link.
Can you help me to learn more?
(Other definitions for hairbrush that I've seen before include "Personal grooming tool" , "Grooming aid" , "It is used with a comb" , "It supplements comb for tidying tresses" , "Locks straightener" .)