It's a good thing (5)
I believe the answer is:
asset
'it's a good thing' is the definition.
(an advantageous or desirable thing)
This is all the clue.
(Other definitions for asset that I've seen before include "Valuable item" , "Valuable thing" , "Tessa (anag)" , "Valuable possession, worth having" , "A possession of some value" .)