Bad horse with a lot to carry (7)
I believe the answer is:
harmful
'bad' is the definition.
(similar in meaning)
'horse with a lot to carry' is the wordplay.
'horse' becomes 'H' (abbreviation as in horse power).
'with' says to put letters next to each other.
'a lot to carry' becomes 'armful' (a lot to carry in one's arms).
'h'+'armful'='HARMFUL'
(Other definitions for harmful that I've seen before include "Injurious" , "Causing injury or damage, say poison or malice" , "Noxious" , "Half rum (anag.)" , "Dangerous, damaging" .)