National hero? Flipping hell! (3,3)
I believe the answer is:
red rum
'national hero?' is the definition.
(horse that won the Grand National three times)
'flipping hell' is the wordplay.
'flipping' shows that the letters should be reversed in order.
'hell' becomes 'murder' (both can mean a terrible experience or situation).
'murder' back-to-front is 'RED RUM'.
(Other definitions for red rum that I've seen before include "Three-time Grand National winner, d. 1995 aged 30" , "famous runner" , "Three times Grand National winner" , "Aintree winner" , "drink" .)