Clarke, 20, impressed on loan at Scottish Premiership rivals Ross County in the first half of the season.
Clarke, who can play full-back or centre-half, "will be a big asset for us," says manager Shaun Maloney.
"Harry has very good technical attributes in both attacking and defensive actions, and I'm really pleased we've been able to secure him," added Maloney.
"He will bring an exciting energy and speed in both areas."
Clarke, who spent last season on loan at Oldham Athletic in English League Two, thanked "amazing club" County for his time in Dingwall in which he scored three goals in 17 appearances.
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