Taps into valuable mineral coloured like rust (5)
I believe the answer is:
ochre
'coloured like rust' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'taps into valuable mineral' is the wordplay.
'taps' becomes 'C H' (I've seen this in other clues).
'into' is an insertion indicator.
'valuable mineral' becomes 'ore' (I've seen this before).
'ch' placed into 'ore' is 'OCHRE'.
Can you help me to learn more?
(Other definitions for ochre that I've seen before include "Earth coloured pigment" , "Earth used in pigments" , "Earth-coloured pigment" , "yellowish-red" , "Yellowish-orange colour" .)