By the sound of it, remained sober (5)
I believe the answer is:
staid
'sober' is the definition.
(synonyms)
'by the sound of it remained' is the wordplay.
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'remained' becomes 'stayed' ('stay' can be a synonym of 'remain').
'stayed' sounds like 'STAID'.
(Other definitions for staid that I've seen before include "Sober, grave" , "Unadventurous, dull" , "Restrained (in character)" , "Calm, respectable and unadventurous" , "Demure" .)