Fully fix outside of rescue vessel having boarded (8)
I believe the answer is:
embarked
'boarded' is the definition.
(embarking is a kind of boarding)
'fully fix outside of' is the wordplay.
'fully' becomes 'ark' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fix' becomes 'embed' (to embed is to fix in place).
'outside of' indicates putting letters inside.
'ark' put into 'embed' is 'EMBARKED'.
'rescue vessel having' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for embarked that I've seen before include "Set out on a voyage" , "Having set sail" , "Went on board ship or plane" , "Began (on project)" , "Got on" .)