A little vegetable in mature plant (5)
I believe the answer is:
agave
'plant' is the definition.
The answer is an example.
'a little vegetable in mature' is the wordplay.
'little' indicates taking the first letters.
'in' indicates putting letters inside.
'mature' becomes 'age' (synonyms).
The first letter of 'vegetable' is 'v'.
'a'+'v'='av'
'av' going into 'age' is 'AGAVE'.
(Other definitions for agave that I've seen before include "native growing up in tropical America" , "Spiky succulent" , "plant in Mexico" , "Succulent; daughter of Cadmus" , "Succulent plant" .)