Criminal Investigation Department filling most of legal document for judge (6)
I believe the answer is:
decide
'for judge' is the definition.
(decide can mean to make a judgment)
'criminal investigation department filling most of legal document' is the wordplay.
'criminal investigation department' becomes 'cid' (this could be a standard abbreviation which I don't know about).
'filling' is an insertion indicator.
'most of' means to remove the last letter.
'legal document' becomes 'deed' (deed is a kind of legal document).
'deed' with its last letter removed is 'dee'.
'cid' going into 'dee' is 'DECIDE'.
Can you help me to learn more?
(Other definitions for decide that I've seen before include "Resolve; give judgement" , "Reach a judgment" , "Make a judgment" , "Adjudicate" , "Reach a conclusion" .)