Join keen beginner and fool (4)
I believe the answer is:
knit
'join' is the definition.
(knitting is a kind of joining)
'keen beginner and fool' is the wordplay.
'beginner' suggests taking the first letters.
'and' is a charade indicator (letters next to each other).
'fool' becomes 'nit' (I've seen this before).
The initial letter of 'keen' is 'k'.
'k'+'nit'='KNIT'
(Other definitions for knit that I've seen before include "Join together (with needles)" , "Draw (brows) together" , "Form by looping and entwining wool etc" , "get together" , "Make a woolly" .)