Cheat with ultimately unsuspecting sweetheart, making bloomer (8)
I believe the answer is:
foxglove
'bloomer' is the definition.
(flower that blooms)
'cheat with ultimately unsuspecting sweetheart' is the wordplay.
'cheat' becomes 'fox' (fox is a kind of cheat).
'with' means one lot of letters go next to another.
'ultimately' suggests the final letters.
'sweetheart' becomes 'love' (both are terms of address to romantic partner).
The last letter of 'unsuspecting' is 'g'.
'fox'+'g'+'love'='FOXGLOVE'
'making' acts as a link.
(Other definitions for foxglove that I've seen before include "Source of 1 dn" , "Plant (of the genus 10 down) with bell-shaped flowers" , "Plants of the genus Digitalis" , "Common wild flower" , "Tall plant with purple or white flowers - vulpine?" .)