Fascinated, when upset network proved an irritation (9)
I believe the answer is:
bewitched
'fascinated' is the definition.
('bewitch' can be a synonym of 'fascinate')
'upset network proved an irritation' is the wordplay.
'upset' is a reversal indicator.
'network' becomes 'web' (synonyms).
'proved an irritation' becomes 'itched' (I can't justify this - if you can you should give a lot more credence to this answer).
'web' reversed gives 'bew'.
'bew'+'itched'='BEWITCHED'
'when' is the link.
Can you help me to learn more?
(Other definitions for bewitched that I've seen before include "film" , "Captivated (by 12?)" , "Fascinated" , "'''..., Bothered and Bewildered'' (Ella Fitzgerald song) (9)'" , "seduced?" .)