Slip up in dumbshow, not ignoring Oscar’s mirth (9)
I believe the answer is:
merriment
'mirth' is the definition.
(thesaurus)
'slip up in dumbshow not ignoring oscar's' is the wordplay.
'slip up' becomes 'err' (to err is to make an error).
'in' means one lot of letters goes inside another.
'dumbshow' becomes 'mime' (a mime gives a wordless performance).
'ignoring' is a deletion indicator.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'not' with 'o' taken away is 'nt'.
'err' placed into 'mime' is 'merrime'.
'merrime'+'nt'='MERRIMENT'
(Other definitions for merriment that I've seen before include "Gaiety, laughter, fun" , "Hilarity" , "laughing maybe" , "Joy" , "Jollity" .)