What peer has, back from old country with retro-style fashion (7)
I believe the answer is:
dukedom
'what peer has' is the definition.
(dukes are peers of the realm)
'back from old country with retro-style fashion' is the wordplay.
'back from' indicates one should take the final letters (the letter at the back of the word).
'country' becomes 'UK' (United Kingdom).
'with' is a charade indicator (letters next to each other).
'retro-style' shows that the letters should be reversed in order (retro means backwards).
'fashion' becomes 'mode' (synonyms).
The last letter of 'old' is 'd'.
'mode' reversed gives 'edom'.
'd'+'uk'+'edom'='DUKEDOM'
(Other definitions for dukedom that I've seen before include "Noble's position" , "Noble rank" , "Title or lands of high ranking nobleman" , "nobility?" , "Hereditary title" .)