Design short shelf filled with endless fish (7)
I believe the answer is:
stencil
'design' is the definition.
(I've seen this before)
'short shelf filled with endless fish' is the wordplay.
'short shelf' becomes 'sil' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'filled with' indicates putting letters inside.
'endless' means to remove the last letter.
'fish' becomes 'tench' (tench is a kind of fish).
'tench' with its final letter taken away is 'tenc'.
'sil' placed around 'tenc' is 'STENCIL'.
Can you help me to learn more?
(Other definitions for stencil that I've seen before include "Text template" , "Sheet with holes used to make ink or paint patterns" , "Clients (anag)" , "Template for pattern" , "Create design" .)