Meal including fillets of tender fish (5)
I believe the answer is:
tetra
'fish' is the definition.
(tetra is a kind of fish)
'meal including fillets of tender' is the wordplay.
'meal' becomes 'tea' (tea is a kind of meal).
'including' is an insertion indicator.
'fillets of tender' becomes 'tr' (I can't explain this - if you can you should give a lot more credence to this answer).
'tea' going around 'tr' is 'TETRA'.
Can you help me to learn more?
(Other definitions for tetra that I've seen before include "With four parts" , "Accosting and detaining (someone)" , "... Four < , in composition" , "rarely seen flower" , "Bright fish; four (prefix)" .)