Minor setback which exposed copper on top of perjury (6)
I believe the answer is:
hiccup
'minor setback' is the definition.
(I've seen this before)
'which exposed copper on top of perjury' is the wordplay.
'exposed' indicates the central letters (outside letters removed).
'copper' becomes 'cu' (chemical symbol).
'on top of' says to take the initial letters.
The middle letters of 'which' are 'hic'.
The initial letter of 'perjury' is 'p'.
'hic'+'cu'+'p'='HICCUP'
(Other definitions for hiccup that I've seen before include "Minor setback" , "minor issue" , "Minor hitch" , "Involuntary spasm of the diaphragm" , "Glitch" .)