Terrible fib initially true (9)

I believe the answer is:

frightful

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'terrible' is the definition.
(I know that terrible can be written as frightful)

'fib initially true' is the wordplay.
'fib initially' becomes 'f' (I can't justify this - if you can you should give a lot more credence to this answer).
'true' becomes 'rightful' (synonyms).
'f'+'rightful'='FRIGHTFUL'

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(Other definitions for frightful that I've seen before include "alarm" , "savage" , "Appalling" , "Terrible, awful" , "nasty" .)

I've seen this clue in The Daily Express.

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