Eccentric prison officer unknown (6)
I believe the answer is:
screwy
'eccentric' is the definition.
(I've seen this before)
'prison officer unknown' is the wordplay.
'prison officer' becomes 'screw' (slang term).
'unknown' becomes 'y' (algebra).
'screw'+'y'='SCREWY'
(Other definitions for screwy that I've seen before include "Slightly mad" , "Odd, eccentric" , "weird" , "Off one's rocker" , "Crazy" .)