Careful cleaner? Unknown (5)
I believe the answer is:
chary
'careful' is the definition.
(I've seen this before)
'cleaner? unknown' is the wordplay.
'cleaner?' becomes 'char' (char is a kind of cleaner).
'unknown' becomes 'y' (algebra).
'char'+'y'='CHARY'
(Other definitions for chary that I've seen before include "Cautious, wary" , "Careful" , "Circumspect" , "cagy" , "Cautiously reluctant" .)