Useless section of welding is rejected (4)
I believe the answer is:
idle
'useless' is the definition.
(I've seen this before)
'section of welding is rejected' is the wordplay.
'section of' indicates the answer is hidden within the clue.
'is rejected' says the letters should be written in reverse.
'eldi' is hidden within 'welding'.
'eldi' reversed gives 'IDLE'.
(Other definitions for idle that I've seen before include "Avoiding work, lazy" , "having no effect" , "Member of comedy team" , "without basis" , "Not in active use" .)