Centre of ruin contains unknown submachine-gun (3)
I believe the answer is:
uzi
'submachine-gun' is the definition.
(I've seen this before)
'centre of ruin contains unknown' is the wordplay.
'centre of' indicates the central letters.
'contains' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
The centre of 'ruin' is 'ui'.
'ui' going around 'z' is 'UZI'.
(Other definitions for uzi that I've seen before include "Type of submachine gun" , "Israeli sub-machine gun" , "kind of weapon" , "killer" , "Submachine-gun" .)