Memorable Beagle's mission? Collecting flora ultimately (8)
I believe the answer is:
haunting
'memorable' is the definition.
(haunting can mean evocative or memorable)
'beagle's mission? collecting flora ultimately' is the wordplay.
'beagle's mission?' becomes 'hunting' (beagles are hunting dogs).
'collecting' is an insertion indicator.
'ultimately' suggests the final letters.
The last letter of 'flora' is 'a'.
'hunting' going around 'a' is 'HAUNTING'.
(Other definitions for haunting that I've seen before include "Visiting as a ghost" , "A show of spirit" , "Poignant, evocative" , "Ghost visitation on ninth Aug" , "Lingering in the mind, like ghost" .)