Doing word (4)
I believe the answer is:
verb
'doing word' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for verb that I've seen before include "A word that conveys action" , "Word that denotes an action or a state" , "Type of word indicating action" , "'In grammar, it denotes action (4)'" , "Part of speech" .)