They also multiply naturally (6)
I believe the answer is:
adders
'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps you can see a link between them that I can't see?
'also multiply naturally' is the wordplay.
'also' becomes 'add' (I've seen this before).
'multiply naturally' becomes 'ers' (I can't explain this - if you can you should give a lot more credence to this answer).
'add'+'ers'='ADDERS'
Can you help me to learn more?
(Other definitions for adders that I've seen before include "Do they deal with tots" , "sum? We do" , "Sadder with snakes" , "Mathematical snakes" , "Totting 25 down?" .)