First to join ends of optical nerve fibre (5)
I believe the answer is:
istle
'fibre' is the definition.
(I've seen this before)
'first to join ends of optical nerve' is the wordplay.
'first to' becomes 'ist' (I've seen this in other clues).
'join' says to put letters next to each other.
'ends of' indicates one should take the final letters (I've seen 'end of' mean this).
The last letters of 'optical nerve' are 'le'.
'ist'+'le'='ISTLE'
(Other definitions for istle that I've seen before include "Fibre from yucca or agave trees" , "for making nets?" , "Carpet material" .)