React badly -- girl's capacity ultimately for deceit (9)
I believe the answer is:
treachery
'deceit' is the definition.
(thesaurus)
'react badly girl's capacity ultimately' is the wordplay.
'badly' is an anagram indicator.
'girl' becomes 'her' (**).
'ultimately' suggests the final letters.
The final letter of 'capacity' is 'y'.
'react' anagrammed gives 'treac'.
'treac'+'her'+'y'='TREACHERY'
'for' acts as a link.
(Other definitions for treachery that I've seen before include "Two-timing" , "Betrayal of trust" , "lack of loyalty" , "perfidious conduct" , "Perfidy" .)