“fell arrest / Without all bail”
'Fell' means fierce or savage (OED fell, adj. 1a: 'cruel, savage, ruthless'). 'Arrest' here is a legal term: a seizure of a person by authority. The poet imagines death as a brutal bailiff or sheriff who carries the body away and accepts no bail — no payment or surety can secure release. Death is the one arrest from which there is no legal remedy.
“My life hath in this line some interest”
'Interest' carries a double meaning: a legal share or claim (OED interest, n., sense 5a: 'a legal concern, title, or right in property'), and the plain sense of significance or importance. 'This line' means both the line of verse the reader holds and the poet's lineage or line of life. The poet is saying that his life has a legal stake — a surviving claim — in this very poem, which will remain with the beloved as a memorial after his death.
historical The sonnet form- TLN 1029biblical allusion
“The earth can have but earth, which is his due”
The line echoes the burial formula drawn from Genesis: God tells Adam that because he sinned, 'dust thou art, and unto dust shalt thou return' (Genesis 3:19, Geneva 1599). The body is made of earth and belongs to the earth as a debt ('his due'); only the spirit is exempt from this repayment. The following line makes the contrast explicit: 'My spirit is thine, the better part of me.'