Canal feature; fasten (4)
I believe the answer is:
I believe this clue is a double definition.
'canal feature' is the first definition.
(I've seen this before)
'fasten' is the second definition.
(I know that lock is a more specific form of the action fasten)
(Other definitions for lock that I've seen before include "element in pack" , "Secure a piece of hair" , "Strand of hair" , "Get stuck in" , "Security device - bit of hair" .)