Company, they say, that is part of network (3)
I believe the answer is:
two
'company they say' is the definition.
(as in 'two's a company, three's a crowd')
'part of network' is the wordplay.
'part of' says the answer is hidden in the clue.
'TWO' is hidden within 'network'.
'that is' is the link.
(Other definitions for two that I've seen before include "Company, so it's said" , "Duo" , "Pair; hour after lunch" , "Cat's coat" , "Pair, deuce" .)