Particular protein-rich food tot tucks into (7)
I believe the answer is:
faddish
'particular' is the definition.
(faddish or faddy can mean picky or particular)
'protein-rich food tot tucks into' is the wordplay.
'protein-rich food' becomes 'fish'.
'tot' becomes 'add' (synonyms).
'tucks into' means one lot of letters goes inside another.
'fish' placed around 'add' is 'FADDISH'.
(Other definitions for faddish that I've seen before include "having peculiar tastes" , "Having many food likes and dislikes" , "Fussy" , "Having personal whims" .)