In need of time out when on the decline (6)
I believe the answer is:
waning
'on the decline' is the definition.
'in need of time out' is the wordplay.
'in need of' becomes 'wanting'.
'time' becomes 't' (abbreviation).
'out' is a deletion indicator.
'wanting' with 't' removed is 'WANING'.
'when' acts as a link.
(Other definitions for waning that I've seen before include "(Of moon) decreasing" , "'Declining, going down (6)'" , "(Of the moon) reducing in apparent size" , "Gradually decreasing in size" , "Fading" .)