Measure mineral endlessly (5)
I believe the answer is:
quart
'measure' is the definition.
(quart is a kind of measure)
'mineral endlessly' is the wordplay.
'mineral' becomes 'quartz' (quartz is a kind of mineral).
'endlessly' means to remove the last letter.
'quartz' with its final letter taken away is 'QUART'.
(Other definitions for quart that I've seen before include "Liquid measure" , "A couple of pints" , "The capacity" , "Two imperial pints" , "water may be measured in this" .)