Irritating person’s old fashioned goodness and cunning (6)
I believe the answer is:
gadfly
'irritating person's' is the definition.
(I've seen this before)
'old fashioned goodness and cunning' is the wordplay.
'old fashioned goodness' becomes 'gad' (old-fashioned way of saying 'goodness!').
'and' is a charade indicator (letters next to each other).
'cunning' becomes 'fly' (fly can mean skilful or sharp).
'gad'+'fly'='GADFLY'
(Other definitions for gadfly that I've seen before include "It bites livestock" , "Persistently annoying person" , "Insect; annoying person" , "Bloodsucker; nuisance" , "Small insect that bites horses and cattle" .)