Bottom of cooling tower is dark (5)
I believe the answer is:
gloom
'is dark' is the definition.
(I've seen this before)
'bottom of cooling tower' is the wordplay.
'bottom of' suggests the final letters.
'tower' becomes 'loom' ('loom' can be a synonym of 'tower'**).
The last letter of 'cooling' is 'g'.
'g'+'loom'='GLOOM'
(Other definitions for gloom that I've seen before include "Darkness; depression" , "Obscurity or melancholy" , "Depression and melancholy" , "Betting figures" , "Murk" .)