Key type added on to register (5)
I believe the answer is:
latch
'key' is the definition.
(thesaurus)
'type added on to register' is the wordplay.
I cannot quite understand how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for latch that I've seen before include "Bar to close" , "Gate fastening" , "Something to secure" , "Gate fastener" , "Door fastener" .)