Criminal pair one found in sect (7)
I believe the answer is:
culprit
'criminal' is the definition.
(I've seen this before)
'pair one found in sect' is the wordplay.
'pair' becomes 'pr'.
'one' becomes 'i' (Roman numeral).
'found in' is an insertion indicator.
'sect' becomes 'cult' (I've seen this in other clues).
'pr'+'i'='pri'
'pri' put into 'cult' is 'CULPRIT'.
(Other definitions for culprit that I've seen before include "The person to blame, the guilty one" , "One perpetrating crime" , "Criminal" , "The one to blame, guilty party" , "one awaiting trial" .)