Coca leaf assortment? Collier may take his pick here (8)
I believe the answer is:
coalface
'collier may take his pick here' is the definition.
(a collier might use a coal pick on a coalface)
'coca leaf assortment?' is the wordplay.
'assortment?' indicates anagramming the letters (I've seen 'assortment' mean this).
'coca'+'leaf'='cocaleaf'
'cocaleaf' is an anagram of 'COALFACE'.
(Other definitions for coalface that I've seen before include "At which miner works" , "Where the action is - in a mine?" , "Part of mine" , "Where the real work takes place!" .)