Unhealthy jerk seals old drawer (5)
I believe the answer is:
toxic
'unhealthy' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'jerk seals old drawer' is the wordplay.
'jerk' becomes 'tic' (jerking movement by the body).
'seals' is an insertion indicator (inserted letters are sealed inside).
'old drawer' becomes 'ox' (I can't justify this - if you can you should give a lot more credence to this answer).
'tic' enclosing 'ox' is 'TOXIC'.
Can you help me to learn more?
(Other definitions for toxic that I've seen before include "word describing bad things" , "Harmful - cox it (anag)" , "Deadly - venomous" , "Poisonous, noxious" .)