Keep hours of darkness for two weeks (9)
I believe the answer is:
fortnight
'two weeks' is the definition.
(synonyms)
'keep hours of darkness' is the wordplay.
'keep' becomes 'fort' (I've seen this before).
'hours of darkness' becomes 'night' (I've seen this before).
'fort'+'night'='FORTNIGHT'
'for' is the link.
(Other definitions for fortnight that I've seen before include "couple of weeks" , "short period?" , "Half a lunar month" , "Period of time" , "Two weeks" .)