Mortar to leave out for each fish (10)
I believe the answer is:
mudskipper
'fish' is the definition.
(mudskipper is a kind of fish)
'mortar to leave out for each' is the wordplay.
'mortar' becomes 'mud' (I can't explain this - if you can you should believe this answer much more).
'to leave out' becomes 'skip' (skipping is a kind of leaving out).
'for each' becomes 'per' (per person).
'mud'+'skip'+'per'='MUDSKIPPER'
Can you help me to learn more?
(Other definitions for mudskipper that I've seen before include "Swimmer" , "Tropical fish that can move over land - risked pump (anag)" .)