Current entering fish grounds (5)
I believe the answer is:
basis
'grounds' is the definition.
(I've seen this before)
'current entering fish' is the wordplay.
'current' becomes 'I' (symbol for electric current).
'entering' means one lot of letters goes inside another.
'fish' becomes 'bass' (bass is a kind of fish).
'i' placed inside 'bass' is 'BASIS'.
(Other definitions for basis that I've seen before include "Underlying support for a theory" , "Foundation or support, or a main principle" , "Groundwork, foundation" , "First principle" , "Foundation, fundament" .)