Grand tower in partial darkness (5)
I believe the answer is:
gloom
'partial darkness' is the definition.
(I've seen this before)
'grand tower' is the wordplay.
'grand' becomes 'G' (G is short for ground as in a thousand).
'tower' becomes 'loom' (synonyms).
'g'+'loom'='GLOOM'
'in' acts as a link.
(Other definitions for gloom that I've seen before include "Dark depression?" , "Betting figures" , "Murk" , "Obscurity or melancholy" , "Darkness; depression" .)