Away three months when business is slack (3,6)
I believe the answer is:
off season
'when business is slack' is the definition.
(I've seen this before)
'away three months' is the wordplay.
'away' becomes 'off' (synonyms).
'three months' becomes 'season' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'off'+'season'='OFF-SEASON'
Can you help me to learn more?
(Other definitions for off season that I've seen before include "less popular period" , "when there's little activity" , "Time of year when usual activity or business is slack" , "Time of year when business is slackest" , "Quiet time of year" .)