Oxford goalkeeper Ryan Clarke produced a man of the match performance to earn manager Michael Appleton a 0-0 draw on his return to old club Portsmouth.Clarke denied Pompey top-scorer Jed Wallace his 15th goal of the season with a brilliant finger-tip save to turn the midfielder's 18-yard effort behind for a corner in the 16th minute.Matt Tubbs looked certain to score three minutes before half time only for Clarke to somehow push his bullet header from a Wallace cross over the crossbar.Pompey, bidding for a fourth successive league win for the first time this season, were comfortably the better team but lacked a killer instinct.Tubbs set the tone for a frustrating afternoon for the hosts by slicing a shot into the stands after only seven minutes at the end of a powerful surge from Wallace.Defender Paul Robinson also headed over from Dan Butler's 31st minute free kick.Danny Hylton's fourth minute long ranger straight into the arms of Pompey goalkeeper Paul Jones was as close as the visitors came to scoring.