By Patrick Johnston SINGAPORE, Jan 28 (Reuters) - World number one Jordan Spieth grabbed a share of the early clubhouse lead at the Singapore Open on Thursday but the American, missing his regular caddie, was left frustrated by some inaccurate approach play. Agent and manager Jay Danzi stepped up to carry the bag on a stifling first morning at the Sentosa Golf Club, with regular caddie Michael Greller taking a rare week off because of injury. The headline act of an otherwise low-key event, Spieth, followed by a few hundred spectators taking advantage of free entry on Thursday, said his bogey-free four-under-par round of 67 which left him level with South Korean world number 26 An Byeong-hun, should have been better. More...