Variety of agate needs working unknown times (4)
I believe the answer is:
onyx
'variety of agate' is the definition.
(I've seen this before)
'working unknown times' is the wordplay.
'working' becomes 'on'.
'unknown' becomes 'y' (algebra).
'times' becomes 'x' (multiplication sign).
'on'+'y'+'x'='ONYX'
'needs' acts as a link.
(Other definitions for onyx that I've seen before include "Stones" , "Kind of agate" , "Black variety of quartz" , "Agate with different layers of colour" , "Cameo material" .)