Investigator mainly took what can be loaded (6)
I believe the answer is:
pistol
'took what can be loaded' is the definition.
I can't judge whether this definition defines the answer.
'investigator mainly' is the wordplay.
'investigator' becomes 'PI' (private investigator).
'mainly' becomes 'stol' (I am not sure about this - if you are sure you should believe this answer much more).
'pi'+'stol'='PISTOL'
Can you help me to learn more?
(Other definitions for pistol that I've seen before include "Type of gun" , "Piece" , "Shakespearean character" , "making an appearance in Shakespeare's King Henry V" , "pop" .)