Fair, on occasion, to regret producing thin thread (8)
I believe the answer is:
filament
'thin thread' is the definition.
(I've seen this before)
'fair on occasion to regret' is the wordplay.
'on occasion' indicates alternate letters.
'to regret' becomes 'lament' (synonyms).
The alternate letters of 'fair' are 'fi'.
'fi'+'lament'='FILAMENT'
'producing' is the link.
(Other definitions for filament that I've seen before include "Thin thread or fibre" , "Thin thread (e.g. in bulb)" , "one can become incandescent" , "It glows in a light bulb" , "Wire in light bulb" .)