Jokes about very strong island birds (7)
I believe the answer is:
puffins
'birds' is the definition.
(puffin is a kind of bird)
'jokes about very strong island' is the wordplay.
'jokes' becomes 'puns' (punning is a kind of joking).
'about' means one lot of letters goes inside another.
'very strong' becomes 'ff' (musical abbreviation for fortissimo, 'very strong').
'island' becomes 'i'.
'ff'+'i'='ffi'
'puns' enclosing 'ffi' is 'PUFFINS'.
(Other definitions for puffins that I've seen before include "Black and white diving birds" , "Bright-billed auks" , "Seabirds with brightly-coloured bills" .)