50 inch thread (5)

I believe the answer is:

lisle

'thread' is the definition.
(I know that lisle is a type of cotton)

'50 inch' is the wordplay.
'50' becomes 'l' (Roman numeral).
'inch' becomes 'isle' (I've seen this before).
'l'+'isle'='LISLE'

(Other definitions for lisle that I've seen before include "Fine cotton thread used in stockings" , "Strong cotton fabric" , "Strong cotton material" , "Cotton stocking thread" , "Fine smooth cotton thread for stockings" .)

I've seen this clue in The Telegraph.

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