Reportedly remained serious (5)
I believe the answer is:
staid
'serious' is the definition.
(I've seen this before)
'reportedly remained' is the wordplay.
'reportedly' indicates a 'sounds like' (homophone) clue.
'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 "Sober, grave" , "composed" , "Dully unadventurous" , "Calm, respectable and unadventurous" , "Unadventurous, dull" .)