Small border for growth in wet ground (5)
I believe the answer is:
sedge
'growth in wet ground' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'small border' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
'for' is the link.
Can you help me to learn more?
(Other definitions for sedge that I've seen before include "Grass-like plant growing in marshes" , "Marsh-dweller" , "Coarse grassy plant" , "Plant like grass" , "Marsh grass" .)