Seated after assembly, calm (6)
I believe the answer is:
sedate
'calm' is the definition.
(I know that calm can be written as sedate)
'seated after assembly' is the wordplay.
'after assembly' indicates anagramming the letters (letters reassembled in a new form).
'seated' is an anagram of 'SEDATE'.
(Other definitions for sedate that I've seen before include "Staid - teased (anag)" , "Calm, not flighty" , "Seated strangely in a demure sort of way" , "Dignified - staid" , "Slow" .)