He said he applied after the Crown Prosecution Service said he would not be prosecuted over the Matrix Churchill affair.
Asked why he was trying to get back into Westminster so soon after leaving it, Mr Clark, 64, replied: 'I thought the recession would end with the election and the party would be tranquil. But there is quite a lot to be done.'
He gave evidence in the trial last November of three directors of the engineering firm Matrix Churchill, accused of breaking arms sanctions on Iraq. After his evidence, the trial collapsed.
Mr Clark was Tory MP for Plymouth Sutton for 18 years until his retirement. The by-election was caused by the death of Judith Chaplin, who retained the Tory seat last year with a 12,357 majority over the Liberal Democrats.