Formal person you can become in posh company? (3)
I believe the answer is:
one
'formal person you' is the definition.
'one' can be an answer for 'you' (I've seen this before). I am unsure of the 'formal person' bit.
I cannot understand how the remainder of the clue works.
(Other definitions for one that I've seen before include "SECOND PART OF 16" , "Hour after midnight" , "1" , "Half of two" , "Unit" .)