Flood’s ending with Noah’s vessel in gloom (4)
I believe the answer is:
dark
'gloom' is the definition.
(thesaurus)
'flood's ending with noah's vessel' is the wordplay.
'ending' says to take the final letters (the ending of the word).
'with' means one lot of letters go next to another.
'noah's vessel' becomes 'ark' (Noah's ark).
The last letter of 'flood' is 'd'.
'd'+'ark'='DARK'
'in' is the link.
(Other definitions for dark that I've seen before include "Dimly lit" , "Closed as theatre" , "Sombre in colour" , "Saturnine" , "Absence of light" .)