Surprisingly mobile blood clots (6)
I believe the answer is:
emboli
'blood clots' is the definition.
(embolus is a kind of blood clot)
'surprisingly mobile' is the wordplay.
'surprisingly' indicates anagramming the letters.
'mobile' is an anagram of 'EMBOLI'.
(Another definition for emboli that I've seen is " Blood clots or air bubbles which cause arterial blockages".)
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