Dull, impact oddly lacking (3)
I believe the answer is:
mat
'dull' is the definition.
('mat' can be similar in meaning to 'dull')
'impact oddly lacking' is the wordplay.
'oddly lacking' means one should take alternating letters.
The alternating letters of 'impact' are 'MAT'.
(Other definitions for mat that I've seen before include "Small rug" , "A piece of material as floor covering" , "Piece of material for wiping feet on" , "Thick flat pad on the floor" , "dull finish" .)