Sail off after British boy (5)
I believe the answer is:
basil
'boy' is the definition.
(I've seen this before)
'sail off after british' is the wordplay.
'off' indicates anagramming the letters.
'after' is a charade indicator (letters next to each other).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'sail' anagrammed gives 'asil'.
'asil' put after 'b' is 'BASIL'.
(Other definitions for basil that I've seen before include "Mediterranean herb" , "Culinary herb" , "Aromatic herb in pesto" , "Aromatic herb with purplish-green leaves" , "Plant; Brush; Fawlty" .)