Pathogen changing shape (8)
I believe the answer is:
heptagon
'shape' is the definition.
(heptagon is a kind of polygon)
'pathogen changing' is the wordplay.
'changing' indicates an anagram.
'pathogen' anagrammed gives 'HEPTAGON'.
(Other definitions for heptagon that I've seen before include "Seven-sided shape" , "Seven-sided object" , "Seven-sided figure" , "A geometric figure with seven sides" .)