Net said to be cast as a substitute (7)
I believe the answer is:
instead
'as a substitute' is the definition.
(I've seen this before)
'net said to be cast' is the wordplay.
'to be cast' indicates an anagram (letters are dispersed or cast around).
'net'+'said'='netsaid'
'netsaid' is an anagram of 'INSTEAD'.
(Other definitions for instead that I've seen before include "as an alternative option" , "In lieu" , "Alternatively" , "As a substitute" , "for a change" .)