Set out with a posh car some day (5)
I believe the answer is:
array
'set out' is the definition.
('array' can be a synonym of 'set out')
'with a posh car some day' is the wordplay.
'with' is a charade indicator (letters next to each other).
'posh car' becomes 'rr' (I've seen this in other clues).
'some day' becomes 'ay' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'rr' is 'arr'.
'arr'+'ay'='ARRAY'
Can you help me to learn more?
(Other definitions for array that I've seen before include "Fine assortment" , "Deck" , "Imposing assembly" , "Matrix; display neatly" , "Orderly arrangement" .)