Hard mineral in estuary (6)
I believe the answer is:
humber
'estuary' is the definition.
(The Humber is an estuary)
'hard mineral' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'mineral' becomes 'umber' (umber is a kind of mineral).
'h'+'umber'='HUMBER'
'in' acts as a link.
(Other definitions for humber that I've seen before include "Hull's river" , "course here" , "Trent/Ouse estuary" , "Yorkshire river" , "Estuary of NE England" .)