Dark period — imminent source of trouble (5)
I believe the answer is:
night
'dark period' is the definition.
(period of the day when it's dark)
'imminent source of trouble' is the wordplay.
'imminent' becomes 'nigh' (I've seen this in another clue).
'source of' says to take the initial letters.
The initial letter of 'trouble' is 't'.
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "Saturday -- Fever (film)" , "Hours of darkness" , "See 9" , "'She walks in beauty, like the . . . . .' (Byron)" , "What proverbially follows day" .)