Reveal the day is nigh (8)
I believe the answer is:
disclose
'reveal' is the definition.
(disclosing is a kind of revealing)
'the day is nigh' is the wordplay.
'the day' becomes 'd'.
'nigh' becomes 'close' ('close' can be similar in meaning to 'nigh').
'd'+'is'+'close'='DISCLOSE'
(Other definitions for disclose that I've seen before include "Divulge, reveal" , "Divulge or expose" , "Make public" , "Report" , "Lay bare" .)