One slicing fish shows main principle (5)
I believe the answer is:
basis
'main principle' is the definition.
(I've seen this before)
'one slicing fish' is the wordplay.
'one' becomes 'i' (Roman numeral).
'slicing' is an insertion indicator (some letters slice into others).
'fish' becomes 'bass' (bass is a kind of fish).
'i' inserted into 'bass' is 'BASIS'.
'shows' acts as a link.
(Other definitions for basis that I've seen before include "grounds" , "Underlying support for a theory" , "Underlying support (for an idea)" , "Fundamental part" , "Reason" .)