They are used for keeping locks in order (5)
I believe the answer is:
combs
'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps you can see an association between them that I can't see?
'used for keeping locks in order' is the wordplay.
'used for keeping' becomes 'cbs' (I can't justify this - if you can you should give a lot more credence to this answer).
'locks in' means one lot of letters goes inside another.
'order' becomes 'om' (Order of Merit).
'cbs' enclosing 'om' is 'COMBS'.
'are' is the link.
Can you help me to learn more?
(Other definitions for combs that I've seen before include "Crests" , "Searches thoroughly" .)