A little treasure is good for them (3)
I believe the answer is:
gem
'a little treasure' is the definition.
(I've seen this in another clue)
'good for them' is the wordplay.
'good' becomes 'g' (abbreviation).
'for' says to put letters next to each other.
'them' becomes 'em' (abbreviation).
'g'+'em'='GEM'
'is' acts as a link.
(Other definitions for gem that I've seen before include "Kindness; comfort (German)" , "Pleasant and cheerful (from German)" , "perfect specimen" , "A jewel or precious stone" , "one of the Cairngorms?" .)