Story of young adult joining navy (4)
I believe the answer is:
yarn
'story' is the definition.
(synonyms)
'young adult joining navy' is the wordplay.
'young' becomes 'y'.
'adult' becomes 'a' (abbreviation).
'joining' is a charade indicator (letters next to each other).
'navy' becomes 'RN' (abbreviation for Royal Navy).
'y'+'a'+'rn'='YARN'
'of' is the link.
(Other definitions for yarn that I've seen before include "Rambling tale" , "Fabricated" , "Spun fibre" , "Rope" , "Tale; weave" .)