Ideal source of relief back aboard a vessel (7)
I believe the answer is:
paragon
'ideal' is the definition.
(I know that paragon is a type of ideal)
'source of relief back aboard a vessel' is the wordplay.
'source of' indicates taking the first letters (the start - or source - of the word).
'back' becomes 'ago' ('years back' means 'years ago').
'aboard' indicates putting letters inside.
'a vessel' becomes 'pan' (vessel or container for cooking).
The first letter of 'relief' is 'r'.
'r'+'ago'='rago'
'rago' placed within 'pan' is 'PARAGON'.
(Other definitions for paragon that I've seen before include "Perfect diamond" , "An ideal" , "Role model" , "Model of excellence or perfection" , "A model as of virtue" .)