Said to be about only half teetotal, but sober (5)
I believe the answer is:
staid
'sober' is the definition.
(synonyms)
'said to be about only half teetotal' is the wordplay.
'to be about' means one lot of letters goes inside another.
'only half' suggests halving the letters.
'teetotal' becomes 'tt' (abbreviation).
'tt' halved is 't'.
'said' placed around 't' is 'STAID'.
'but' is the link.
(Other definitions for staid that I've seen before include "Sedate; dull" , "Sedate and serious" , "Sedate, decorous" , "Conventional (character)" , "Dully unadventurous" .)