International computer system's not hard, in my opinion (5)
I believe the answer is:
tenet
'my opinion' is the definition.
(tenet can mean a view or opinion)
'international computer system's not hard' is the wordplay.
'international computer system' becomes 'the net' (the internet).
'not' indicates named letters should be taken away.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'thenet' with 'h' taken out is 'TENET'.
'in' is the link.
(Other definitions for tenet that I've seen before include "Belief - maxim" , "Dogma or principle" , "Key principle" , "Doctrine, dogma, belief" , "A religious doctrine proclaimed as true without proof" .)