Vessel whose sister ships were named Niña and Pinta (5,5)
I believe the answer is:
santa maria
'vessel' is the definition.
(I have seen 'Historic vessel' mean 'santa maria' so perhaps 'vessel' could also mean 'santa maria')
'sister ships were named nina and pinta' is the wordplay.
I cannot really see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'were' could be 'sat' (sitting is a kind of being) and 'sat' is found in the remaining letters.
This may be the basis of the clue (or it may be nonsense).
'whose' is the link.
Can you help me to learn more?
(Other definitions for santa maria that I've seen before include "Columbus' ship" , "Columbus's flagship" , "Brazilian city" , "Flagship of Columbus" , "Historic vessel" .)