Worthless, having little impact, mostly (3)
I believe the answer is:
bum
'worthless' is the definition.
(I've seen this in other clues)
'little impact mostly' is the wordplay.
'little impact' becomes 'bump' (I've seen this before).
'mostly' means to remove the last letter.
'bump' with its last letter taken away is 'BUM'.
'having' is the link.
(Other definitions for bum that I've seen before include "Behind" , "False" , "Begging" , "American hobo or English bottom" , "Cadge" .)