High spirits as police impound posh car -- mine's broken down! (9)
I believe the answer is:
merriment
'high spirits' is the definition.
(thesaurus)
'police impound posh car mine's broken down' is the wordplay.
'police' becomes 'met' (I've seen this in other clues**).
'impound' means one lot of letters goes inside another.
'posh car' becomes 'rr' (I've seen this in other clues).
'broken down' indicates anagramming the letters.
'mine' anagrammed gives 'imen'.
'rr'+'imen'='rrimen'
'met' going around 'rrimen' is 'MERRIMENT'.
'as' acts as a link.
(Other definitions for merriment that I've seen before include "Hilarity" , "laughing maybe" , "Gaiety, laughter, fun" , "Jollity" , "Joy" .)