Admiral's second vessel is obscure (4)
I believe the answer is:
dark
'is obscure' is the definition.
(similar in meaning)
'admiral's second vessel' is the wordplay.
'admiral's second' becomes 'd' (rnd letter of 'admiral').
'vessel' becomes 'ark' (ark is a kind of vessel).
'd'+'ark'='DARK'
(Other definitions for dark that I've seen before include "Black, perhaps - night" , "Absence of light" , "Saturnine" , "Gloomy" , "Closed as theatre" .)