Said to have remained sober (5)
I believe the answer is:
staid
'sober' is the definition.
(synonyms)
'said to have remained' is the wordplay.
'to have' is an insertion indicator.
'remained' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'said' enclosing 't' is 'STAID'.
Can you help me to learn more?
(Other definitions for staid that I've seen before include "Very conventional and dull" , "Proper; sedate" , "Serious or sober in character" , "Stuffy, traditional" , "Sedate, unadventurous" .)