Commodore shortly sat when so made up (8)
I believe the answer is:
composed
'made up' is the definition.
(composing is a kind of making up)
'commodore shortly sat' is the wordplay.
'commodore shortly' becomes 'com' (I can't explain this - if you can you should believe this answer much more).
'sat' becomes 'posed' ('pose' can be a synonym of 'sit').
'com'+'posed'='COMPOSED'
'when so' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for composed that I've seen before include "Written" , "Calm and collected" , "Field did" , "Serenely self-possessed" , "arranged" .)