Reveal that day is nigh (8)
I believe the answer is:
disclose
'reveal' is the definition.
(disclosing is a kind of revealing)
'day is nigh' is the wordplay.
'day' becomes 'd'.
'nigh' becomes 'close' ('close' can be similar in meaning to 'nigh').
'd'+'is'+'close'='DISCLOSE'
'that' acts as a link.
(Other definitions for disclose that I've seen before include "Sold ices (anag)" , "Make (secret) known" , "Let out" , "Make known - subject is closed" , "Divulge or expose" .)