Buttonhole the fellow further down (6)
I believe the answer is:
flower
'buttonhole' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'the fellow further down' is the wordplay.
'the fellow' becomes 'f' (abbreviation - of a society etc.).
'further down' becomes 'lower' (I've seen this before).
'f'+'lower'='FLOWER'
Can you help me to learn more?
(Other definitions for flower that I've seen before include "River" , "rose, perhaps" , "Garden plant" , "Plant cultivated for blossoms" , "prime of life" .)