Ironing gown and formal suit (7,5)
I believe the answer is:
evening dress
'formal suit' is the definition.
(formal clothing)
'ironing gown' is the wordplay.
'ironing' becomes 'evening' (ironing evens out creases in clothes).
'gown' becomes 'dress' (gown is a kind of dress).
'evening'+'dress'='EVENING-DRESS'
'and' is the link.
(Other definitions for evening dress that I've seen before include "Formal dinner wear" , "DJ" , "Formal attire" .)