A diminutive figure, but growing (4)
I believe the answer is:
weed
'growing' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'a diminutive figure' is the wordplay.
'a diminutive' becomes 'wee' (I've seen this in another clue).
'figure' becomes 'd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'wee'+'d'='WEED'
'but' is the link.
Can you help me to learn more?
(Other definitions for weed that I've seen before include "gone!" , "Plant growing in the wrong place!" , "Ineffectual person" , "Went" , "nuisance in bed" .)