Argue with two bishops about nightmare (7)
I believe the answer is:
bugbear
'nightmare' is the definition.
(thesaurus)
'argue with two bishops about' is the wordplay.
'two bishops' means two replacements for 'bishop'.
'with' says to put letters next to each other.
'bishop' becomes 'B' (abbreviation used in chess).
'bishop' becomes 'B' (abbreviation used in chess).
'about' indicates anagramming the letters.
'b'+'b'='bb'
'argue' put after 'bb' is 'bbargue'.
'bbargue' is an anagram of 'BUGBEAR'.
(Other definitions for bugbear that I've seen before include "Cause of great distress" , "bogeyman" , "Particular irritation" , "[BOGYMAN]" , "Cause of irritation" .)