Day off - very good, that is (6)
I believe the answer is:
sickie
'day off' is the definition.
'good that is' is the wordplay.
'good' becomes 'sick' (I can't justify this - if you can you should believe this answer much more).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'sick'+'ie'='SICKIE'
'very' acts as a link.
Can you help me to learn more?
(Other definitions for sickie that I've seen before include "ill-deserved leave?" , "Day off for malingerer" , "Leave taken with feigned illness" , "In day off work" .)