Female is tackling adult about Oscar's mess (6)
I believe the answer is:
fiasco
'mess' is the definition.
(thesaurus)
'female is tackling adult about oscar's' is the wordplay.
'female' becomes 'f' (abbreviation).
'tackling' indicates putting letters inside (to tackle is to take possession of).
'adult' becomes 'a' (abbreviation).
'about' becomes 'c' (abbreviation for 'circa').
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'is' going around 'a' is 'ias'.
'f'+'ias'+'c'+'o'='FIASCO'
(Other definitions for fiasco that I've seen before include "Calamity" , "Complete disaster" , "Utter and ridiculous failure" , "Embarrassing failure" , "Ignominious result" .)