1. fell: inhumanly cruel; lethal; dire; sinister.   that fell arrest: i.e., death.

3. My life hath in this line some interest: i.e., my life has some ownership rights to this poetry. —I'm guessing that the other owner of the poetry is the poet's beloved, who inspired the poetry.

4. still: always.

6. part was: part of me which was.

9. dregs: the basest or least desirable portion.

11. the coward conquest of a wretch's knife: i.e., the poet's corpse. —The idea seems to be that death (the "wretch") is a coward who conquers by sneaking up and stabbing is victims in the back.

12. of thee to be remembered: to be remembered by you.

13. that: the body.

14. that: i.e., the "that which it contains" of the previous line.   this: i.e., the poet's spirit embodied in his poetry.