Not a seat left for duo tossed into mail boat (6,3)
I believe the answer is:
packed out
'not a seat left' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'duo tossed into mail boat' is the wordplay.
'tossed' indicates anagramming the letters.
'into' indicates putting letters inside.
'mail boat' becomes 'packet' (synonyms).
'duo' is an anagram of 'dou'.
'dou' put inside 'packet' is 'PACKED OUT'.
'for' acts as a link.
Can you help me to learn more?
(Another definition for packed out that I've seen is " (Hall) full to capacity".)