Advanced alongside second class property (5)
I believe the answer is:
asset
'property' is the definition.
(I've seen this before)
'advanced alongside second class' is the wordplay.
'advanced' becomes 'a'.
'alongside' says to put letters next to each other.
'second' becomes 's' (common abbreviation).
'class' becomes 'set' (I've seen this before).
'a'+'s'+'set'='ASSET'
(Other definitions for asset that I've seen before include "Valuable possession, worth having" , "Someone or something of value" , "Something well worth having" , "Help" , "Useful or valuable property" .)