A particular dark time to sustain a casual relationship (3-5,5)
I believe the answer is:
one-night stand
'a casual relationship' is the definition.
'a particular dark time to sustain' is the wordplay.
'a particular' becomes 'one'.
'dark time' becomes 'night' (I've seen this before).
'to sustain' becomes 'stand'.
'one'+'night'+'stand'='ONE-NIGHT STAND'
(Other definitions for one-night stand that I've seen before include "Very brief fling" , "Very short "relationship"" , "little time for love" , "Very short-term affair" , "Very brief affair" .)