Trendy one on about apartment and increase in prices (9)
I believe the answer is:
inflation
'increase in prices' is the definition.
(I've seen this before)
'trendy one on about apartment' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'one' becomes 'i' (Roman numeral).
'about' means one lot of letters go next to another.
'apartment' becomes 'flat' (synonyms).
'i'+'on'='ion'
'ion' put after 'flat' is 'flation'.
'in'+'flation'='INFLATION'
'and' acts as a link.
(Other definitions for inflation that I've seen before include "General fall in value of money" , "Building costs" , "General increase in prices" , "Increasing costs" , "getting increasingly aired?" .)