Impossibility: tin thrown up seen flying down to earth (2-8)
I believe the answer is:
no-nonsense
'down to earth' is the definition.
I can't judge whether this definition defines the answer.
'impossibility tin thrown up seen flying' is the wordplay.
'impossibility' becomes 'nono' (I've seen this before).
'tin' becomes 'Sn' (Sn is the chemical symbol for tin).
'thrown up' shows that the letters should be reversed in order (in a down clue, letters go up).
'flying' indicates an anagram (letters fly around into new positions).
'sn' backwards is 'ns'.
'seen' anagrammed gives 'ense'.
'nono'+'ns'+'ense'='NO-NONSENSE'
Can you help me to learn more?
(Other definitions for no-nonsense that I've seen before include "Sensible and straightforward" , "Simple, sensible" , "Straightforward, practical" , "Simple, straightforward" , "Down-to-earth" .)