Mineral left in box with iron lid (7)
'mineral' is the definition.
(felspar is a kind of mineral)
'left in box with iron lid' is the wordplay.
'left in' becomes 'l' (I've seen this in other clues).
'box' becomes 'spar' (sparring is a kind of boxing).
'with' is a charade indicator (letters next to each other).
'iron lid' becomes 'fe' (I can't explain this - if you can you should give a lot more credence to this answer).
'lspar' after 'fe' is 'FELSPAR'.
Can you help me to learn more?
(Other definitions for felspar that I've seen before include "Abundant mineral", "Rock-forming mineral".)