Crossword Genius

Large quill pens key (6)

Ross

I believe the answer is:

spline

'key' is the definition.
(I've seen this before)

'large quill pens' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'quill' becomes 'spine' (I've seen this in another clue).
'pens' means one lot of letters goes inside another (inserted letters are held or penned in).
'l' going into 'spine' is 'SPLINE'.

(Another definition for spline that I've seen is " Key to locate wheel on a shaft".)

I've seen this clue in The Independent.
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