Calm rascal assembled true rabble (13)
I believe the answer is:
imperturbable
'calm' is the definition.
(synonyms)
'rascal assembled true rabble' is the wordplay.
'rascal' becomes 'imp' (synonyms).
'assembled' indicates an anagram.
'true'+'rabble'='truerabble'
'truerabble' is an anagram of 'erturbable'.
'imp'+'erturbable'='IMPERTURBABLE'
(Other definitions for imperturbable that I've seen before include "Stoical" , "Equanimous" , "Not excitable" , "Unflappable" , "Unruffled" .)