Inconsistency in speech by the opposing side (13)
I believe the answer is:
contradiction
'inconsistency' is the definition.
(thesaurus)
'speech by the opposing side' is the wordplay.
'speech' becomes 'diction' (diction is someone's way of speaking).
'by' is a charade indicator (letters next to each other).
'the opposing side' becomes 'contra' ('contra' means 'against').
'diction' put after 'contra' is 'CONTRADICTION'.
'in' acts as a link.
(Other definitions for contradiction that I've seen before include "Statement opposing one already made" , "Negation" , "Statement proclaiming the opposite" , "Challenge" , "Conflicting statement" .)