MacLennan is a product of Clyde"s youth system. Born in Livingston, MacLennan signed a YTS contract with Clyde in the summer of 2006, and made his début on the opening day of that season, coming on as a substitute in a 0–0 draw against Street Johnstone. He was given a three-year professional contract in June 2007.
MacLennan hit another milestone against Street Johnstone the following season, when he scored his first senior goal in a 1–0 victory in August 2007.
He notched his second goal for Clyde in the Scottish Challenge Cup, in a defeat at home to Dunfermline Athletic. His other notable goals include the late equalizer against Greenock Morton in the 2–1 away victory in which Clyde scored two injury time goals to seal a very important league victory at Cappielow, and a double in a 3–2 victory over Livingston in April 2008.
MacLennan was shown his first senior red card in a Scottish First Division defeat away to Livingston, where he was judged to have committed a professional foul. He is the older brother of football player Roddy MacLennan, who he also played with at Clyde and currently plays with at Linlithgow Rose.
In a Scottish Cup tie against Montrose in November 2007, the MacLennans became the first brothers to play for Clyde for 17 years.
MacLennan"s contract was terminated in June 2009, following Clyde"s relegation and financial troubles.