Audibly remained calm (5)
I believe the answer is:
staid
'calm' is the definition.
(I've seen this before)
'audibly remained' is the wordplay.
'audibly' indicates a 'sounds like' (homophone) clue.
'remained' becomes 'stayed' (staying is a kind of remaining).
'stayed' sounds like 'STAID'.
(Other definitions for staid that I've seen before include "Sedate, sober" , "Sedate, unadventurous" , "Conventional (character)" , "Grave, sedate" , "Sedate and serious" .)