But Danny Kerry's team looked out of sorts as an attacking Argentine side left the British defence bamboozled as they opened up a 2-0 lead.
Britain picked up the pace in the second half at the Olympic Green Hockey Stadium and after a series of instinctive saves Thomas finally made the breakthrough.
Clewlow levelled the match in the 51st minute but Argentina were only kept at bay by some solid keeping from Beth Storry.
Argentina missed a chance to claim a late victory but Rosario Lucjetti failed to get on the end of a pass to set up a clear goal-scoring opportunity to the relief of the British side.
Great Britain now face a make-or-break match against New Zealand on Thursday before final matches against Japan and the US.
Kerry said: "We had played pretty well and Argentina had been very good at their penalty corners.
"We tidied up one thing and it was really talking about the next score and generating momentum."
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