Deep blue mineral, quartz, unlimited supply (11)
I believe the answer is:
ultramarine
'deep blue' is the definition.
(I've seen this before)
'mineral quartz unlimited supply' is the wordplay.
'unlimited' says to take the centre (discard the edges or 'limits' of the word).
'supply' indicates anagramming the letters (supply of letters to make).
The middle letters of 'quartz' are 'uart'.
'mineral'+'uart'='mineraluart'
'mineraluart' is an anagram of 'ULTRAMARINE'.
(Other definitions for ultramarine that I've seen before include "Tone" , "Blue pigment originally made from lapis lazuli" , "Vivid blue" , "Brilliant deep blue" , "See a colour" .)