What's this, in a nutshell? (5)
I believe the answer is:
acorn
'nutshell?' is the definition.
The answer and definition can be both plants as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'what's this in a' is the wordplay.
'what's this' becomes 'cor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'a' becomes 'an'.
'cor' placed within 'an' is 'ACORN'.
Can you help me to learn more?
(Other definitions for acorn that I've seen before include "form of seeding" , "from which something bigger may develop" , "Nut of oak tree" , "fruit in a cup" , "Potential oak" .)