One tucks into fish heart (5)
I believe the answer is:
basis
'heart' is the definition.
(thesaurus)
'one tucks into fish' is the wordplay.
'one' becomes 'I' (Roman numeral).
'tucks into' is an insertion indicator.
'fish' becomes 'bass' (bass is a kind of fish).
'i' placed within 'bass' is 'BASIS'.
(Other definitions for basis that I've seen before include "Foundation of a theory or process" , "grounds" , "Groundwork, foundation" , "Form of support" , "Fundamental part" .)