Uniform in East Germany Wessis regularly wear for parties (7,5)
I believe the answer is:
evening dress
'wear for parties' is the definition.
'evening dress' can be an answer for 'wear' (I have seen 'Formal wear' mean 'evening dress' so perhaps 'wear' could also mean 'evening dress'). I'm unsure of the 'for parties' bit.
'uniform in east germany wessis regularly' is the wordplay.
'uniform' becomes 'even' (uniform can mean even or level).
'in east germany' becomes 'ingdr' (I can't justify this - if you can you should give a lot more credence to this answer).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'wessis' are 'ess'.
'even'+'ingdr'+'ess'='EVENING-DRESS'
Can you help me to learn more?
(Other definitions for evening dress that I've seen before include "Formal dinner wear" , "DJ" , "Formal attire" .)