11/21 across
Sad
saint
may
be
found
in
Derbyshire
cave
(4,4)
Sad saint may be found in Derbyshire cave (4,4)
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The whole clue is the definition (& lit or double definition).
How the clue works
Derbyshire cave is your definition — it's the name of a famous cavern (and the distinctive fluorite mineral found there). Sad gives you BLUE (as in feeling blue), and saint points to JOHN, so put them together and you've got your answer.