The 18-year-old centre-back, who will stay at Fratton Park until January, has featured in three of Pompey’s pre-season friendlies and scored in the 2-0 win at Woking.
Clarke told Pompey’s official website: “I’m delighted to be here. I’ve already played a few games, so it’s great to have all the paperwork signed.
“I’ve enjoyed the matches that I’ve been involved in so far and have done alright, so now I want to improve from there.
“I spent a lot of time on the bench last season and just got a few minutes here and there, so I wanted to go out and hopefully get some more regular football.”
Clarke progressed through Ipswich’s academy and has made five first-team appearances after making his debut at Crawley in the Capital One Cup last August.