I perhaps alarm nun with more misbehaving (5,7)
I believe the answer is:
roman numeral
'i' is the definition.
(I is the Roman numeral for one)
'alarm nun with more misbehaving' is the wordplay.
'with' means one lot of letters go next to another.
'misbehaving' indicates anagramming the letters.
'alarm'+'nun'+'more'='alarmnunmore'
'alarmnunmore' anagrammed gives 'ROMAN NUMERAL'.
'perhaps' acts as a link.
(Other definitions for roman numeral that I've seen before include "I say" , "Such as V, X or L" , "Eg, X or V or M" , "I for one!" , "I, perhaps" .)