A girl came in to study the weapon (6)
I believe the answer is:
cannon
'weapon' is the definition.
(cannon is a kind of weapon)
'a girl came in' is the wordplay.
'a girl' becomes 'ann'.
'came' becomes 'con' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'ann' placed within 'con' is 'CANNON'.
'to study the' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for cannon that I've seen before include "The large gun sounds like a church dignitary" , "Field gun" , "Big gun" , "Caught girl on piece of artillery" , "Large artillery piece" .)