23 down
Small
telescope
of
superior
quality
includes
detachable
cap
(6)
Small telescope of superior quality includes detachable cap (6)
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The definition is at the beginning of the clue.
How the clue works
The definition is Small telescope — a finder is indeed a small sighting telescope. Take of superior quality, which gives you FINER, then "includes" tells you to pop in D, the cap (first letter) of "detachable", slotting it in to give you FIN-D-ER.