Couple out of love cheerful after one relatively dark period (8)
I believe the answer is:
twilight
'one relatively dark period' is the definition.
(time of day)
'couple out of love cheerful' is the wordplay.
'couple' becomes 'two' (a couple can mean two of something).
'out' means to remove the last letter.
'of love' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'cheerful' becomes 'light'.
'two' with its final letter taken off is 'tw'.
'tw'+'i'+'light'='TWILIGHT'
'after' is the link.
(Other definitions for twilight that I've seen before include "when the night's still young?" , "Condition of decline" , "period towards the end of the day?" , "Period of illumination when sun is just below horizon" , "Period of gradual decline or obscurity" .)