What can destroy wall, or hold bricks together? (6)
I believe the answer is:
This clue is a double definition.
'what can destroy wall' is the first definition.
(mortar is a type of gun)
'hold bricks together?' is the second definition.
(to mortar is to fix bricks together with mortar)
'or' acts as a link.
(Other definitions for mortar that I've seen before include "Weapon - bonding material" , "Gun; brick bonder" , "Lime mixture; a cannon" , "Gun; brick bond" , "Bomb-firing weapon" .)