Introduction of naval vessel that's not so old (5)
I believe the answer is:
newer
'not so old' is the definition.
('new' can be the opposite of 'old' so perhaps 'newer' is reasonable for 'not so old')
'introduction of naval vessel' is the wordplay.
'introduction of' says to take the initial letters.
'vessel' becomes 'ewer' (ewer is a type of jug).
The initial letter of 'naval' is 'n'.
'n'+'ewer'='NEWER'
'that's' acts as a link.
(Other definitions for newer that I've seen before include "More current" , "Fresher, not as old" , "Fresher or more novel" , "More pristine or recently invented" , "More up-to-date" .)