Flett, 28, began his career with the youth team of Bradford City, and was awarded a first-team squad number in February 2011, and later made his debut in May of the same year.
The midfielder joined Cleethorpes Town in January 2012, and was appointed the club's captain in 2017.
He said: "I’m looking forward to getting going and being around the lads after a long time out."
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