Infect badly after man becomes harmful (10)
I believe the answer is:
maleficent
'harmful' is the definition.
(I've seen this before)
'infect badly after man' is the wordplay.
'badly' indicates anagramming the letters.
'after' is a charade indicator (letters next to each other).
'man' becomes 'male'.
'infect' with letters rearranged gives 'ficent'.
'ficent' put after 'male' is 'MALEFICENT'.
'becomes' is the link.
(Other definitions for maleficent that I've seen before include "Evil" , "Harmful" .)