"15 by the sound of it"

This is a homophone clue - look for indicators like 'we hear', 'sounds like', 'say', 'aloud', or 'reportedly' - the answer sounds like another word

Join the wordplay parts together in sequence

Answer: PATOIS | Definition: '15 by the sound of it' | Wordplay: PA (father, “Old man”) + TO (from surface) + IS (island)