Engineers returned with unknown mineral (5)
I believe the answer is:
emery
'mineral' is the definition.
(I know that emery is a type of mineral)
'engineers returned with unknown' is the wordplay.
'engineers' becomes 'reme' (Royal Electrical and Mechanical Engineers).
'returned' shows that the letters should be reversed in order.
'with unknown' becomes 'y' (algebra. I am not sure about the 'with' bit.).
'reme' written backwards gives 'emer'.
'emer'+'y'='EMERY'
Can you help me to learn more?
(Other definitions for emery that I've seen before include "Abrasive form of corundum" , "Dick - - , late comedian" , "Hard grey-black mineral used as an abrasive" , "old comedian" , "Abrasive powder" .)