One trapped in fish grounds (5)
I believe the answer is:
basis
'grounds' is the definition.
(I've seen this before)
'one trapped in fish' is the wordplay.
'one' becomes 'i' (Roman numeral).
'trapped in' means one lot of letters goes inside another.
'fish' becomes 'bass' (bass is a kind of fish).
'i' placed into 'bass' is 'BASIS'.
(Other definitions for basis that I've seen before include "Ground, foundation" , "Foundation or support, or a main principle" , "grounds" , "First principle" , "Principle on which a theory is founded" .)