Sober, but said to be out to start trouble (5)
I believe the answer is:
staid
'sober' is the definition.
('staid' can be a synonym of 'sober')
'said to be out to start trouble' is the wordplay.
'to be out' indicates putting letters inside.
'to start' suggests taking the first 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" .)