Possible Clues:

• Add one litre, tap turning
• Add words into
• Insert a note written by crime fighters
• Insert (between other terms of a series)
• Misleadingly, perhaps, put in policing organisation, getting knocked back
• Law enforcers worried to act as go-between?
• Insert — not a reptile (anag)
• Put in cross-border force at start of enquiry
• Crime-fighters scoffed, seeing insert
• Multinational force worried, being put in the middle