Determination shown by Marine aboard excellent vessel (8)
I believe the answer is:
firmness
'determination' is the definition.
(thesaurus)
'marine aboard excellent vessel' is the wordplay.
'marine' becomes 'rm'.
'aboard' means one lot of letters goes inside another.
'excellent' becomes 'fine' (synonyms).
'vessel' becomes 'ss'.
'rm' put inside 'fine' is 'firmne'.
'firmne'+'ss'='FIRMNESS'
'shown by' acts as a link.
(Other definitions for firmness that I've seen before include "Rigidity" , "Resolution" , "Solidity" , "Strength of will" .)