The most common and most likely answer to this crossword clue is the 6 letter word MORTAR
I've seen this crossword clue in The Times
M
O
R
T
A
R
More clues leading to the result MORTAR
- Short cannon’s middle-of-the-road rating
- Mixture used in building gun
- What can destroy wall, or hold bricks together?
- . Artillery item capable of absorbing crushing force?
- River to fill moat damaged by king’s gun firing shells
- Hawk carries this weapon
- Roman’s capsized sailor’s vessel
- Bomb scrap left in ruin
- May help in building a house, or destroying it
- Fatal, having ultimately changed sides to get weapon
Buddy explains!

The straight part
We know that "What" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what = mortar.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MORTAR mean?
MORTAR noun- A mixture of lime or cement, sand and water used for bonding building blocks.
- A muzzle-loading, indirect fire weapon with a tube length of 10 to 20 calibers and designed to lob shells at very steep trajectories.
- A hollow vessel used to pound, crush, rub, grind or mix ingredients with a pestle.
- To use mortar or plaster to join two things together.
- To pound in a mortar.
- To fire a mortar (weapon).