Insect that’s always flourished in marshy borders (6)
I believe the answer is:
mayfly
'insect' is the definition.
(I know that mayfly is a type of ephemerid)
'always flourished in marshy borders' is the wordplay.
'always' becomes 'ay' (archaic term for 'always').
'flourished' becomes 'fl' (I've seen this in other clues).
'in' is an insertion indicator.
'borders' suggests removing the centre (only the edge or border letters remain).
'marshy' with its centre removed is 'my'.
'ay'+'fl'='ayfl'
'ayfl' inserted inside 'my' is 'MAYFLY'.
'that's' acts as a link.
(Other definitions for mayfly that I've seen before include "Short-lived insect with delicate wings" , "Ephemeral insect" , "Short-lived spring insect" .)