Accessory with evening dress: one must get into function (7)
I believe the answer is:
adjunct
'accessory' is the definition.
('adjunct' can be a synonym of 'accessory')
'evening dress one must get into function' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'un' (I've seen this in other clues) and 'un' is present in the answer.
This explanation may well be incorrect...
'with' is the link.
Can you help me to learn more?
(Other definitions for adjunct that I've seen before include "Thing added" , "Person subordinate to another" , "Something attached but not essential" , "Incidental extra" , "Something joined or added" .)