Thus, to me, that's not all (4)
I believe the answer is:
some
'not all' is the definition.
('some' is the opposite to 'all')
'thus to me' is the wordplay.
'thus' becomes 'so' (synonyms).
'so'+'me'='SOME'
'that's' is the link.
(Other definitions for some that I've seen before include "Unknown or unspecified amount or number" , "An unspecified quantity, not all" , "Several" , "An undetermined number" , "Undefined number" .)