Poisonous beef consumed by little jerk (5)
I believe the answer is:
toxic
'poisonous' is the definition.
(synonyms)
'beef consumed by little jerk' is the wordplay.
'beef' becomes 'ox'.
'consumed by' means one lot of letters goes inside another (inserted letters get consumed).
'little jerk' becomes 'tic' (jerking movement by the body).
'ox' going into 'tic' is 'TOXIC'.
(Other definitions for toxic that I've seen before include "Deadly - venomous" , "Harmful - cox it (anag)" , "Poisonous, noxious" , "word describing bad things" .)