It's an extra good thing (5)
I believe the answer is:
bonus
'it's an extra' is the definition.
(bonus can mean extra or additional)
'good thing' is the wordplay.
'good' becomes 'bon' (I've seen this in other clues).
'thing' becomes 'us' (I can't explain this - if you can you should give a lot more credence to this answer).
'bon'+'us'='BONUS'
Can you help me to learn more?
(Other definitions for bonus that I've seen before include "Extra payment for good performance" , "Extra payment for good results" , "- generis means unique" , "Extra payment, may be performance-related" , "Additional wage" .)
