When speaking, remained sober (5)
I believe the answer is:
staid
'sober' is the definition.
(synonyms)
'when speaking remained' is the wordplay.
'when speaking' shows a homophone (sound like).
'remained' becomes 'stayed' ('stay' can be a synonym of 'remain').
'stayed' is a homophone of 'STAID'.
(Other definitions for staid that I've seen before include "Sedate, unadventurous" , "Grave, sedate" , "Stuffy, traditional" , "Sober, grave" , "Conventional (character)" .)