Almost mistaken conclusion by a listener (4)
I believe the answer is:
near
'almost' is the definition.
(thesaurus)
'mistaken conclusion by a listener' is the wordplay.
'conclusion' indicates one should take the final letters.
'by' says to put letters next to each other.
'a listener' becomes 'ear' (I've seen this before).
The final letter of 'mistaken' is 'n'.
'n'+'ear'='NEAR'
(Other definitions for near that I've seen before include "Mean, close" , "Not far distant in time or space etc." , "Close in time or space" , "Close; parsimonious" , "Close, not far away" .)