"This one is a groundnut", I put in (4)
I believe the answer is:
unit
'this one is a groundnut i put in' is the definition.
'unit' can be an answer for 'one' (synonyms). I am not sure about the remainder of the definition.
'one is a groundnut i put in' is the wordplay.
'one' becomes 'un' (northern English dialect form of 'one').
'is a groundnut' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'put in' is an insertion indicator.
'un'+'t'='unt'
'unt' enclosing 'i' is 'UNIT'.
Can you help me to learn more?
(Other definitions for unit that I've seen before include "Constituent" , "A considerable number of soldiers" , "army group" , "Small group" , "Kitchen fitment" .)