16M covers how to set the clock mid-game where the tournament director (or anybody else) supplies one.
Curiously, I’m having trouble finding a rule which states that a director has the right to put a clock on a game where the players don’t have one (or have decided to play without a clock). There also doesn’t seem to be a rule which states that if a clock becomes available to a player mid-game, that he has the right to set and start it mid-game, the game having started without. Actually, I’m having trouble finding a rule that states that it is obligatory to play with a clock if one is available.
Having said all that, there is also no rule which states directly that you have to play a chess game with a board and pieces, rather than “blindfold”. All these things are too obvious to mention, perhaps.