Great thinker, intense I suspect (8)
I believe the answer is:
einstein
'great thinker' is the definition.
(Albert Einstein, the famous physicist)
'intense i suspect' is the wordplay.
'suspect' indicates an anagram (a suspicious or unusual spelling).
'intense'+'i'='intensei'
'intensei' with letters rearranged gives 'EINSTEIN'.
(Other definitions for einstein that I've seen before include "one foreign genius" , "Physicist who formulated theories of relativity" , "Albert, who founded the theory of relativity" , "Scientist - seen in it (anag)" , "German-born physicist" .)