Consent expresses nothing (3,2)
I believe the answer is:
say-so
'consent' is the definition.
'expresses nothing' is the subsidiary indication.
'expresses' becomes 'says' (saying is a kind of expressing).
'nothing' becomes 'o' (looks like zero - 0).
'says'+'o'='sayso'
(Other definitions for say-so that I've seen before include "Go-ahead" , "Authority - rumour" , "Final approval" , "Authorisation" , "Authority to make a decision" .)