Mayonnaise gets lower rating (8,4)
I believe the answer is:
dressing down
'rating' is the definition.
Although both the answer and definition are present participle verbs, I don't see how one could define the other.
'mayonnaise gets lower' is the wordplay.
'mayonnaise' becomes 'dressing' (I can't justify this - if you can you should believe this answer much more).
'gets' means one lot of letters go next to another.
'lower' becomes 'down' (synonyms).
'dressing'+'down'='DRESSING-DOWN'
Can you help me to learn more?
(Other definitions for dressing down that I've seen before include "using low gear?" , "telling-off" , "rebuke" , "Maybe not wearing a tie in the office" , "Wearing jeans possibly" .)