Breaking seal, catch sight of revolutionary mineral (6)
I believe the answer is:
gypsum
'mineral' is the definition.
(gypsum is a kind of mineral)
'breaking seal catch sight of revolutionary' is the wordplay.
'breaking' means one lot of letters goes inside another.
'seal' becomes 'gum' (I've seen this before).
'catch sight of' becomes 'spy' (spying is a kind of catching sight of).
'revolutionary' is a reversal indicator.
'spy' reversed gives 'yps'.
'gum' placed around 'yps' is 'GYPSUM'.
(Other definitions for gypsum that I've seen before include "Mineral giving plaster of Paris" , "Mineral used in making plaster of Paris" , "Soft white or grey mineral" , "Plaster of Paris is made from this chalk-like mineral" , "Ingredient of plaster of Paris" .)