Lord installs square safe (5)
I believe the answer is:
peter
'safe' is the definition.
(slang term for a safe)
'lord installs square' is the wordplay.
I cannot really see how this works, but
'square' could be 't' (a T-Square is a technical drawing instrument) and 't' is found within the answer.
'lord' could be 'peer' (peer is a kind of lord) and 'peer' is located in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for peter that I've seen before include "Play high card before low" , "foundation for church" , "Cephas" , "Leader of the Apostles" , "prison cell" .)