Blow up a canal (8)
I believe the answer is:
uppercut
'blow' is the definition.
(uppercut is a kind of blow)
'up a canal' is the wordplay.
'a' becomes 'per'.
'canal' becomes 'cut' (cut is a kind of canal).
'up'+'per'+'cut'='UPPERCUT'
(Other definitions for uppercut that I've seen before include "Boxing punch delivered vertically" , "Swinging blow in boxing" , "Boxer's blow" , "Blow from fist to chin" , "A vertical punch to chin delivered by boxer" .)