Speak fondly about carbon plant (4)
I believe the answer is:
coco
'plant' is the definition.
The answer is an example.
'speak fondly about carbon' is the wordplay.
'speak fondly' becomes 'coo' (to coo is to speak gently or fondly).
'about' indicates putting letters inside.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'coo' placed around 'c' is 'COCO'.
(Other definitions for coco that I've seen before include "French fashion designer, who branched into perfume etc" , "address for Chanel" , "'... the Clown, ... Chanel (4)'" , "Famous clown" , "Gabrielle Bonheur Chanel" .)