He values a vessel having an arrangement of roses (8)
I believe the answer is:
assessor
'he values' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a vessel having an arrangement of roses' is the wordplay.
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'having' means one lot of letters go next to another.
'an arrangement of' indicates an anagram.
'roses' anagrammed gives 'essor'.
'a'+'ss'+'essor'='ASSESSOR'
Can you help me to learn more?
(Other definitions for assessor that I've seen before include "One who estimates value etc for insurance company" , "Person valuing property" , "One who rates for tax" , "One values property" , "Valuer, estimator" .)