Feller’s tool unknown by a European (3)
I believe the answer is:
axe
'feller's tool' is the definition.
(I've seen this in another clue)
'unknown by a european' is the wordplay.
'unknown' becomes 'x' (algebra).
'by' is a charade indicator (letters next to each other).
'european' becomes 'E' (abbreviation e.g. EU).
'x' after 'a' is 'ax'.
'ax'+'e'='AXE'
(Other definitions for axe that I've seen before include "Cancel" , "Chopping tool with heavy blade" , "Fire weapon" , "Remove ruthlessly or suddenly" , "flower in Devon" .)