Unlike a bikini formerly covering European bird (3-5)
I believe the answer is:
one-piece
'unlike' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'a bikini formerly covering european bird' is the wordplay.
'a bikini formerly' becomes 'once' (I can't justify this - if you can you should believe this answer much more).
'covering' indicates putting letters inside.
'european' becomes 'e' (abbreviation e.g. EU).
'bird' becomes 'pie' (sort of bird).
'e'+'pie'='epie'
'once' going around 'epie' is 'ONE-PIECE'.
Can you help me to learn more?
(Other definitions for one-piece that I've seen before include "Bathing-costume" , "Made as a single item" , "(E.g. swimming costume) not as separates" , "Made as a single unit" .)