Grim person achieving things, we hear (4)
I believe the answer is:
dour
'grim' is the definition.
(I know that grim can be written as dour)
'person achieving things we hear' is the wordplay.
'person achieving things' becomes 'doer' (someone doing things).
'we hear' indicates a 'sounds like' (homophone) clue.
'doer' sounds like 'DOUR'.
(Other definitions for dour that I've seen before include "Uncompromising" , "Stern-looking, forbidding" , "Dreary" , "Stern, humourless" , "Stubborn" .)