Scientist worked out that I'm anaemic (13)
I believe the answer is:
mathematician
'scientist' is the definition.
(mathematician is a kind of scientist)
'worked out that i'm anaemic' is the wordplay.
'worked out' indicates anagramming the letters.
'that'+'im'+'anaemic'='thatimanaemic'
'thatimanaemic' anagrammed gives 'MATHEMATICIAN'.
(Other definitions for mathematician that I've seen before include "Number specialist" , "student engaged with combinatorics?" , "Eg, Leonhard Euler" , "Euclid, say" , "Studier of organisms and their surroundings" .)