Tricky to wash gravel through subsoil layer (7)

I believe the answer is:

hardpan

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'gravel through subsoil layer' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'tricky to wash' is the wordplay.
'tricky' becomes 'hard' (similar in meaning).
'to' means one lot of letters go next to another.
'wash' becomes 'pan' (panning is a kind of washing).
'hard'+'pan'='HARDPAN'

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