One entering fish grounds (5)
I believe the answer is:
basis
'grounds' is the definition.
(I've seen this before)
'one entering fish' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entering' means one lot of letters goes inside another.
'fish' becomes 'bass' (bass is a kind of fish).
'i' going within 'bass' is 'BASIS'.
(Other definitions for basis that I've seen before include "Groundwork, foundation" , "grounds" , "Reason" , "Foundation, fundament" , "Principle on which a theory is founded" .)