Henry, unknown doctor with a monster (5)
I believe the answer is:
hydra
'monster' is the definition.
(I know that Hydra is a type of mythical creature)
'henry unknown doctor with a' is the wordplay.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'unknown' becomes 'y' (algebra).
'doctor' becomes 'dr' (abbreviation).
'with' means one lot of letters go next to another.
'h'+'y'+'dr'+'a'='HYDRA'
(Other definitions for hydra that I've seen before include "Nine-headed monster of Greek myth" , "Multi-headed monster slain by Hercules" , "Many-headed snake slain by Hercules" , "Monster Hercules slew" , "M any-headed monster fought by Hercules" .)