It's a good thing there's a second collection (5)
I believe the answer is:
asset
'it's a good thing' is the definition.
(an advantageous or desirable thing)
'there's a second collection' is the wordplay.
'there's' is a charade indicator (letters next to each other).
'second' becomes 's' (common abbreviation).
'collection' becomes 'set' (set is a kind of collection).
'a' put next to 's' is 'as'.
'as'+'set'='ASSET'
(Other definitions for asset that I've seen before include "Part of motor vehicle wheel" , "Holding" , "Useful or valuable property" , "Plus point" , "Something useful" .)