Mineral component of fossil I catalogued (6)
I believe the answer is:
silica
'mineral' is the definition.
(silica is a kind of mineral)
'component of fossil i catalogued' is the wordplay.
'component of' indicates a hidden word (I've seen this in other clues).
'SILICA' is hidden in the letters of 'fossil i catalogued'.
(Other definitions for silica that I've seen before include "Hard colourless compound from rocks" , "Main constituent of sand" , "Constituent of sandstone and quartz" , "Glassy substance in quartz and sand" , "Common base of rock" .)