Blow up theatre put up by canal (8)
I believe the answer is:
uppercut
'blow' is the definition.
(uppercut is a kind of blow)
'up theatre put up by canal' is the wordplay.
'theatre' becomes 'rep' (I've seen this in other clues**).
'put up' says the letters should be written backwards.
'by' says to put letters next to each other.
'canal' becomes 'cut' (cut is a kind of canal).
'rep' backwards is 'per'.
'up'+'per'+'cut'='UPPERCUT'
(Other definitions for uppercut that I've seen before include "Boxing punch delivered vertically" , "Swinging blow from below aimed at an opponent's chin" , "Boxer's blow" , "Type of punch" , "Swinging blow directed at an opponent's chin" .)