Mother has say - that's excellent (4)
I believe the answer is:
mega
'excellent' is the definition.
(old-fashioned UK slang for 'very good')
'mother has say' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'has' means one lot of letters goes inside another.
'say' becomes 'eg' (both can mean 'for example').
'ma' placed around 'eg' is 'MEGA'.
'that's' is the link.
(Other definitions for mega that I've seen before include "ace" , "wicked" , "Informally great or large" , "Game (anag) -- great" , "Very large (colloq.)" .)