Approaching Turkish capital in darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'approaching turkish capital' is the wordplay.
'approaching' becomes 'nigh' (as in 'the end is nigh').
'capital' indicates taking the first letters.
The initial letter of 'turkish' is 't'.
'nigh'+'t'='NIGHT'
'in' is the link.
(Other definitions for night that I've seen before include "See 8" , "- School, - owl" , "Thing (anag)" , "See 1 Across" , "Time for turning" .)