Unusually mobile, small mass causes obstruction of blood vessel (8)
I believe the answer is:
embolism
'obstruction of blood vessel' is the definition.
'embolism' can be an answer for 'obstruction' (I have seen 'Arterial obstruction ' mean 'embolism' so perhaps 'obstruction' could also mean 'embolism'). I am not sure about the 'of blood vessel' bit.
'unusually mobile small mass' is the wordplay.
'unusually' indicates anagramming the letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'mobile' anagrammed gives 'emboli'.
'emboli'+'s'+'m'='EMBOLISM'
'causes' acts as a link.
Can you help me to learn more?
(Other definitions for embolism that I've seen before include "Blood vessel blockage" , "obstructive clot" , "Obstruction of a blood vessel as by clot or bubble" , "Obstruction of artery by blood clot" , "Blockage of an artery" .)