Obstruction demanded to protect part of canal (8)
I believe the answer is:
blockade
'obstruction' is the definition.
(blockade is a kind of obstruction)
'demanded to protect part of canal' is the wordplay.
'demanded' becomes 'bade' (as bade is the past tense of bid).
'to protect' is an insertion indicator (inserted letters are protected or taken in).
'part of canal' becomes 'lock' ('lock' is part of 'canal').
'bade' enclosing 'lock' is 'BLOCKADE'.
(Other definitions for blockade that I've seen before include "The sealing off a place to prevent entry or exist" , "Isolation of place by sea" , "Military restriction of access" , "Prevent(ion of) access and exit" , "Action taken to prevent entry as of goods" .)