Keystone Cops' initial material, worth mining (4)
I believe the answer is:
core
'keystone' is the definition.
(thesaurus)
'cops' initial material worth mining' is the wordplay.
'initial' indicates taking the first letters.
'material worth mining' becomes 'ore' (ores are mined).
The initial letter of 'cops' is 'c'.
'c'+'ore'='CORE'
(Other definitions for core that I've seen before include "Inmost part" , "Central part of an apple" , "Central zone" , "Central or most important part of something" , "essential element" .)