Numbers it's not odd to see in a ship (6)
I believe the answer is:
sevens
'numbers' is the definition.
(seven is a kind of number)
'not odd to see in a ship' is the wordplay.
'not odd' becomes 'even' ('even' is the opposite to 'odd').
'to see in' means one lot of letters goes inside another.
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'even' put within 'ss' is 'SEVENS'.
'it's' is the link.
(Other definitions for sevens that I've seen before include "rugby competition" , "Form of Rugby Union" , "cards!" , "77" , "Numbers" .)