Pair stood up to being duplicitous (3,5)
I believe the answer is:
two faced
'being duplicitous' is the definition.
(I've seen this before)
'pair stood up to' is the wordplay.
'pair' becomes 'two' (a pair means two of something).
'stood up to' becomes 'faced' (face can mean to confront or stand up to).
'two'+'faced'='TWO-FACED'
(Other definitions for two faced that I've seen before include "Unreliable" , "double?" , "deceitful" , "Janus" , "Duplicitous" .)