Small border of growth (5)
I believe the answer is:
sedge
'growth' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'small border' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for sedge that I've seen before include "Coarse, grasslike plant" , "Plant growing in swampy ground" , "Bog plant; type of warbler" , "Plant like grass" , "Grass-like plant growing in wet places" .)