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' means to look at the middle letters.
'contains' indicates putting letters inside.
'unknown' becomes 'z' (represents an unknown value in algebra).
The middle 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" .)