Just sat on by female, that's terrible (9)

I believe the answer is:

frightful

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'terrible' is the definition.
(thesaurus)

'just sat on by female' is the wordplay.
'just' becomes 'rightful' ('rightful' can be similar in meaning to 'just').
'sat on' is a charade indicator (letters next to each other).
'by female' becomes 'f' (abbreviation. I am not sure about the 'by' bit.**).
'rightful' put after 'f' is 'FRIGHTFUL'.

'that's' is the link.

Can you help me to learn more?

(Other definitions for frightful that I've seen before include "ghastly" , "savage" , "very bad" , "horrible" , "from hell" .)

I've seen this clue in The Independent.

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