It holds up writer that's useless (5)
I believe the answer is:
inept
'useless' is the definition.
(I've seen this in another clue)
'it holds up writer' is the wordplay.
'holds' means one lot of letters goes inside another.
'up' is a reversal indicator (in down clue: letters go upwards).
'writer' becomes 'pen' (something which writes).
'pen' backwards is 'nep'.
'it' enclosing 'nep' is 'INEPT'.
'that's' is the link.
(Other definitions for inept that I've seen before include "Futile" , "Lousy" , "Incompetent or awkward" , "Not competent" , "lacks skill" .)