Sober, but said to be out to start trouble (5)
I believe the answer is:
staid
'sober' is the definition.
(synonyms)
'said to be out to start trouble' is the wordplay.
'to be out' means one lot of letters goes inside another.
'to start' says to take the initial letters.
The first letter of 'trouble' is 't'.
'said' enclosing 't' is 'STAID'.
'but' is the link.
(Other definitions for staid that I've seen before include "Stuffy, traditional" , "Sedate, sober" , "Serious or sober in character" , "Sedate, decorous" , "composed" .)