Catch a blow on the back of the foot (4)
I believe the answer is:
trap
'catch' is the definition.
(I know that trap is a type of catch)
'a blow on the back of the foot' is the wordplay.
'a blow' becomes 'rap' (rap is a kind of blow).
'on' is a charade indicator (letters next to each other).
'the back of' says to take the final letters.
The last letter of 'foot' is 't'.
'rap' after 't' is 'TRAP'.
(Other definitions for trap that I've seen before include "Cakehole (slang)" , "The Web" , "Light carriage - it's sometimes sprung!" , "Part of a snare" , "Golfing hazard" .)