I’d lost out, being calm (6)
I believe the answer is:
stolid
'being calm' is the definition.
(thesaurus)
'i'd lost out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'id'+'lost'='idlost'
'idlost' anagrammed gives 'STOLID'.
(Other definitions for stolid that I've seen before include "Showing little or no interest or emotion" , "Not excitable or emotional" , "Heavy" , "displaying little emotion" , "Emotionally insensitive" .)