Part of a gun, we hear, to make a gap in the defences (6)
I believe the answer is:
breach
'make a gap in the defences' is the definition.
I can't judge whether this defines the answer.
'part of a gun we hear' is the wordplay.
'part of a gun' becomes 'breech' (I've seen this before).
'we hear' indicates a 'sounds like' (homophone) clue.
'breech' is a homophone of 'BREACH'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for breach that I've seen before include "Break (in a wall, etc)" , "Non-observance" , "Opening - quarrel" , "The breaking of a law or agreement" , "Fissure" .)