Dressing lacks core and is tasteless (6)
I believe the answer is:
garish
'tasteless' is the definition.
(similar in meaning)
'dressing lacks core and is' is the wordplay.
I cannot really understand how this works, but
'is' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for garish that I've seen before include "Bright and showy" , "Gaudy, vulgarly bright" , "Loud" , "Colourful in a vulgar manner" , "Gaudy and tasteless" .)