Mystery pong bearable after muggy day (6,4)
I believe the answer is:
closed book
'mystery' is the definition.
(I've seen this before)
'pong bearable after muggy day' is the wordplay.
'pong' becomes 'BO' (body odour).
'bearable' becomes 'ok' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'after' means one lot of letters go next to another.
'muggy' becomes 'close'.
'day' becomes 'd' (abbreviation for day).
'bo'+'ok'='book'
'close'+'d'='closed'
'book' put after 'closed' is 'CLOSED BOOK'.
Can you help me to learn more?
(Other definitions for closed book that I've seen before include "What loss-making turf accountant did" , "Subject about which one knows nothing" , "Mystery person" , "Took no more bets" , "Enigma" .)