Be still to sit for another portrait (6)
I believe the answer is:
repose
'be still' is the definition.
(thesaurus)
'sit for another portrait' is the wordplay.
'sit' becomes 'pose' ('pose' can be a synonym of 'sit').
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'another portrait' becomes 're' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pose' put after 're' is 'REPOSE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for repose that I've seen before include "Rest - tranquillity" , "peace" , "Relaxation" , "place" , "Lie at rest" .)