A bit of welding up is not working (4)
I believe the answer is:
idle
'is not working' is the definition.
(I've seen this before)
'a bit of welding up' is the wordplay.
'a bit of' says the answer is hidden in the clue (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'up' is a reversal indicator (in down clue: letters go upwards).
'eldi' is hidden in the letters of 'welding'.
'eldi' backwards is 'IDLE'.
(Other definitions for idle that I've seen before include "Without work" , "Hang about doing nothing" , "Loaf" , "Avoiding work, lazy" , "Lazy, doing nothing" .)