But it's not a weapons training establishment! (7,2,4)
I believe the answer is:
college of arms
'but it's not a' is the definition.
I can't judge whether this definition defines the answer.
'weapons training establishment' is the wordplay.
I cannot really see how this works, but
'weapons' could be 'arms' ('arm' can be a synonym of 'weapon') and 'arms' is located in the answer.
'establishment' could be 'college' (college is a kind of establishment) and 'college' is present in the answer.
The remaining letters 'of' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for college of arms that I've seen is " Heraldic authority".)