Frowning on balloon expedition - important item forgotten! (7,8)
I believe the answer is:
'important item forgotten' is the definition.
I can't tell whether this definition defines the answer.
'frowning on balloon expedition' is the wordplay.
'frowning on' becomes 'glaring' (I can't explain this - if you can you should give a lot more credence to this answer).
'balloon' becomes 'O' (O resembles a balloon).
'expedition' becomes 'mission' (a mission or expedition to accomplish something).
Can you help me to learn more?