This uses charade (word building) combined with deletion. Charade means joining separate word parts together, while deletion involves removing specific letters from a word.
Look for a phrase meaning 'part of detective force', then find a word where you remove the first letter (indicated by 'first off'). These two parts join together to make the answer.
Answer: INCIDENT | IN CID (part of detective force) + [s]ENT (sent with first letter removed) = INCIDENT (disturbance)