Plainly distressed but retains composure (2,5)
I believe the answer is:
in tears
'plainly distressed' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'retains composure' is the wordplay.
'retains' can be anagrammed to 'IN TEARS'.
Though, I'm unsure how this is indicated.
'but' is the link.
Can you help me to learn more?
(Other definitions for in tears that I've seen before include "Weeping" , "sadly" , "Very unhappy" , "Maudlin" , "Crying" .)