Remained steady when upset (6)
I believe the answer is:
stayed
'remained' is the definition.
('stay' can be a synonym of 'remain')
'steady when upset' is the wordplay.
'when upset' indicates an anagram.
'steady' with letters rearranged gives 'STAYED'.
(Other definitions for stayed that I've seen before include "Remained, propped up" , "Kept going - or didn't" , "Secured; remained" , "Didn't go" , "Remained, did not leave" .)