Peculiar upset involving wayward act of heroism (7-2)
I believe the answer is:
derring-do
'act of heroism' is the definition.
'peculiar upset involving wayward' is the wordplay.
'peculiar' becomes 'odd' (I've seen this before).
'upset' shows that the letters should be reversed in order.
'involving' means one lot of letters goes inside another.
'wayward' becomes 'erring'.
'odd' back-to-front is 'ddo'.
'ddo' placed around 'erring' is 'DERRING-DO'.
(Other definitions for derring-do that I've seen before include "Literary word for heroic courage or action" , "Courageous action" , "Brave acts" , "Swashbuckling boldness" , "Brave and heroic feat(s)" .)