Jacob rashly holding something poisonous (5)
I believe the answer is:
cobra
'something poisonous' is the definition.
'jacob rashly holding' is the wordplay.
'holding' indicates a hidden word.
'COBRA' is hidden in the letters of 'jacob rashly'.
(Other definitions for cobra that I've seen before include "Poisonous snake" , "Something with a hood" , "Reptile (found in Cabinet Office Briefing Room A?)" , "Highly venomous hooded reptile" , "Hooded snake - AC model of old sports car" .)