Clapp made his senior debut in county cricket for the Sussex Cricket Board in a List A one-day match against Herefordshire at Colwall in the 1st round of the 2000 NatWest Trophy; with Sussex winning the match, he subsequently appeared in the following round against Berkshire at Hastings, which Sussex lost.
[1] In 2000, he was offered a one-year contract extension by Sussex, alongside Paul Havell, Matt Prior, and Michael Yardy.
[2] Having played for the Sussex Second XI in 3-day matches since 1999, Clapp made his first-class debut for Sussex against Leicestershire at Horsham in the 2002 County Championship;[3] he credited his first-class debut to Les Lenham, who helped Clapp to improve his batting in the nets during the winter of 2001–02.
[4] He made a single one-day appearance for Sussex, against the touring West Indies A cricket team at Hove in 2002.
[7] In May 2008, he made the highest individual score in the Sussex Cricket League by a Worthing batsman, when he made 200 not out at Manor Road against Pulborough Cricket Club.