DJ is one equalizing deck (7,5)
I believe the answer is:
evening dress
'dj' is the definition.
(I've seen this before)
'one equalizing deck' is the wordplay.
'one equalizing' becomes 'evening' (I can't justify this - if you can you should believe this answer much more).
'deck' becomes 'dress' (I've seen this before).
'evening'+'dress'='EVENING-DRESS'
'is' is the link.
Can you help me to learn more?
(Other definitions for evening dress that I've seen before include "Formal attire" , "Formal dinner wear" , "DJ" .)