Swears loudly twice opening case of endives (4)

'swears' is the definition.
Although both the answer and definition are verbs in their -s form, I can't understand how they can define each other.

'loudly twice opening case of endives' is the wordplay.
'loudly' becomes 'f' (short for forte - 'loud' in music).
'twice' indicates the doubling of some letters.
'opening' indicates putting letters inside (some letters open a gap in others).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'endives' with its centre taken out is 'es'.
'f' duplicated is 'ff'.
'ff' placed inside 'es' is 'EFFS'.

Can you help me to learn more?