Clinton Friend, in New Job, Keeps His Power
Thomas F McLarty 3d, pushed out as Pres Clinton's chief of staff more than three years ago, still wields as much influence as he did while member of Cabinet; lifelong friend of Pres Clinton belongs to tiny elite who can walk into Oval Office, and even into President's private living quarters, unscheduled and unannounced; has made nearly 40 trips to Latin America as envoy for President, and has emerged as diplomatic troubleshooter elsewhere; has become publicist for Clinton's campaign to win Congressional authority to negotiate new trade deals, and has facilitated White House meetings with big-donor business executives--activity that has recently made him target of campaign finance investigators; photo (M)