Expensive stamp posh Paddy collects (8)
(Other definitions for upmarket that I've seen before include "Suitable for affluent consumers" , "For the better off" , "Demanding higher prices" , "for wealthy consumers" , "Aimed at those with money" .)
'expensive' is the definition.
(I've seen this before)
'stamp posh paddy collects' is the wordplay.
'stamp' becomes 'mark' (I can't justify this - if you can you should believe this answer much more).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'paddy' becomes 'pet' (I've seen this in other clues).
'collects' means one lot of letters goes inside another.
'u'+'pet'='upet'
'mark' placed within 'upet' is 'UP-MARKET'.
Can you help me to learn more?
(Other definitions for up-market that I've seen before include "Sold to more affluent clientele" , "Classy" , "Towards more expensive tastes" , "High in price or prestige" , "Towards the more expensive end of goods" .)