Mining Derbyshire pit he thought to provide descriptive phrase (7)
I believe the answer is:
epithet
'descriptive phrase' is the definition.
(I've seen this before)
'mining derbyshire pit he thought to provide' is the wordplay.
'to provide' indicates the answer is hidden within the clue.
'EPITHET' is hidden in the letters of 'mining derbyshire pit he thought'.
(Other definitions for epithet that I've seen before include "The 23: an -- for Richard I" , "Appellation" , "Sobriquet" , "Fitting description" , "Descriptive term used to express a real quality" .)