'Express' really go-ahead (3-2)
I believe the answer is:
say-so
'go-ahead' is the definition.
(I've seen this before)
'express really' is the wordplay.
'express' becomes 'say' (saying is a kind of expressing).
'really' becomes 'so' (both can mean 'extremely').
'say'+'so'='SAY-SO'
(Other definitions for say-so that I've seen before include "Mere assertion (colloq.)" , "Authority to make or allow something to happen" , "Go-ahead" , "Authority to make a decision" , "Arbitrary assertion" .)