The striker has gone back to Fir Park six years after he was sold to Bristol City in a £700,000 deal in 2009.
He has since spent time with their rivals Bristol Rovers after a loan switch to Brentford and was at Dundee last season, scoring in each of his first eight games for the club.
Twice-capped Scotland international Clarkson was freed when his contract expired after netting only once more thereafter and he feels he has learned a lot since he first left Lanarkshire.
The 29-year-old said: “It’s great to get back this year and finally sign. The important thing for me was to get in and do my pre-season with a club I’ve signed up for.
“I was young and had not a care in the world the last time I was here so now that I’m back, I’m a wiser head.
“I would like to think if anyone needs a helping hand or a quiet word, I can help them in that situation.”
Clarkson will look to play a part in taking Motherwell back up the SPFL Premiership table after they needed a play-off with Rangers to stay in the top flight last season.
They had finished 11th in the standings after coming second in 2014 and claiming a place in the Europa League qualifying rounds.
Clarkson added: “There’s always a positive atmosphere about the club. Everybody wants to put right what happened last year.
“It wasn’t the way everyone probably expected it to go after having the highs of finishing in the top half. There’s an urge there and drive now to make things right.”