[yom]

SetPlayerName

And more precisely, something like that.

pawn Code:

new PlayerName[MAX_PLAYER_NAME];
GetPlayerName(playerid, PlayerName, MAX_PLAYER_NAME);
format(PlayerName, sizeof PlayerName, "[%s]%s", cmdtext[6], PlayerName); //assuming command is "/clan", cmdtext[6] is the start of the command argument. 
SetPlayerName(playerid, PlayerName);