Clarke's 90th minute header came from an unrehearsed set-piece.
"We have not worked on the long throws from the right-hand side so that was a bonus," added Tilson.
"Peter does get in good positions and that is his second goal this season. But on another day we could have lost by four or five goals.
"If they had scored early on then we would have had to open up and possibly got beaten by a hatful.
"But it is a great result for us against a team that will finish in the top two of the Premier League I'm sure."
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Luiz Felipe Scolari suggested that his Chelsea team lacked confidence in front of goal having created several chances in the tie.
"I need to teach to my players more concentration when they arrive in front of goal," said the manager, ahead of the replay on 14 January.
"Maybe it is confidence when they arrive in front of goal. They need to score more and get more confidence. We played very well and created 15 chances but only one goal."
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