The term REPLY TO A KNOCK is a phrase used to describe the action of responding to someone knocking on a door. The crossword solutions for this term are ENTER (5 letters), COMEIN (6 letters), ITSOPEN (7 letters), and WHOISIT (7 letters). ENTER is a verb meaning to go into or arrive at a place, while comein is a command to ENTER a place. Itsopen is an adjective meaning that something is not closed or locked, and whoisit is a question asking who is at the door.