Gun poised, is not as good as one used to be (5,2,3)

'gun poised is not' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'as good as one used to be' is the wordplay.
I cannot really see how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
'good' could be 'g' (abbreviation) and 'g' is found within the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'to' is within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?