Convenient to adopt unsuitable name for fibre (8)
I believe the answer is:
filament
'for fibre' is the definition.
(filament is a kind of fibre)
'convenient to adopt unsuitable name' is the wordplay.
'convenient' becomes 'fit' (fit can mean convenient or appropriate).
'to adopt' indicates putting letters inside.
'unsuitable' becomes 'lame' (lame can mean feeble or unsuitable).
'name' becomes 'n' (common abbreviation e.g. on forms).
'lame'+'n'='lamen'
'fit' placed around 'lamen' is 'FILAMENT'.
(Other definitions for filament that I've seen before include "It glows in a light bulb" , "Wire in incandescent bulb" , "Thin thread or fibre" , "Thin thread (e.g. in bulb)" , "One may be incandescent" .)