Not quite prevent firm being loathsome (10)

I believe the answer is:

detestable

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'loathsome' is the definition.
(thesaurus)

'not quite prevent firm being' is the wordplay.
'not quite' means to remove the last letter.
'prevent' becomes 'deter' (I've seen this before).
'firm being' becomes 'stable' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'deter' with its last letter taken off is 'dete'.
'dete'+'stable'='DETESTABLE'

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(Other definitions for detestable that I've seen before include "Abhorrent" , "Loathsome" .)

I've seen this clue in The Telegraph.

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