Crossword Genius

A deadly weapon held in a left-open entrance. Terrible! (10)

Ross

I believe the answer is:

abominable

I'm a little stuck... Click here to teach me more about this clue!

'terrible' is the definition.
(thesaurus)

'a deadly weapon held in a left-open entrance' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'in' is present in the answer.
'left' could be 'l' (common abbreviation) and 'l' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for abominable that I've seen before include "Odious" , "Very bad, terrible" , "Hateful" , "Atrocious" , "Causing moral revulsion" .)

I've seen this clue in the Sydney Morning Herald.
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