Poison concealed by seven ombudsmen (5)
I believe the answer is:
venom
'poison' is the definition.
(I know that venom is a type of animal toxin)
'concealed by seven ombudsmen' is the wordplay.
'concealed by' says the answer is hidden in the clue.
'VENOM' can be found hidden inside 'seven ombudsmen'.
(Other definitions for venom that I've seen before include "Virulence" , "Snake poison" , "bitterness" , "Poison; malice" , "Snake's poison" .)