Trendy supplement found in salmon, for example (7)
I believe the answer is:
faddish
'trendy' is the definition.
(I've seen this in another clue)
'supplement found in salmon for example' is the wordplay.
'supplement' becomes 'add' (I've seen this in another clue).
'found in' indicates putting letters inside.
'salmon for example' becomes 'fish' (salmon is a kind of fish).
'add' inserted into 'fish' is 'FADDISH'.
(Other definitions for faddish that I've seen before include "Having many food likes and dislikes" , "Fussy" , "Having personal whims" , "having peculiar tastes" .)