Doctor X, vegan animal rights activist (4)
I believe the answer is:
moby
'vegan animal rights activist' is the definition.
(musician and animal rights activist Moby)
'doctor x' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'x' becomes 'by' (both can mean multiplied by).
'mo'+'by'='MOBY'
(Other definitions for moby that I've seen before include "A phone" , "First name of the White Whale" , "Herman Melville's great novel '. . . . Dick'" , "Herman Melville wrote '. . . . Dick'" , "and 20 Across: Herman Melville novel" .)