Where it's unclear, agree to differ about chap that's upset (4,4)
I believe the answer is:
grey area
'where it's unclear' is the definition.
(a grey area is an area or topic of ambiguity)
'agree to differ about chap that's upset' is the wordplay.
'to differ' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'chap' becomes 'ray' (man's name).
'that's upset' is a reversal indicator.
'ray' in reverse letter order is 'yar'.
'agree' with letters rearranged gives 'greea'.
'greea' placed around 'yar' is 'GREY AREA'.
(Other definitions for grey area that I've seen before include "It's hard to describe precisely" , "One of undefined boundaries" , "subject to debate?" , "Ill-defined region" , "Situation or topic not clearly understood by eager Ray" .)