Merest trace of canker in flatter flower (9)
I believe the answer is:
buttercup
'flatter flower' is the definition.
'buttercup' can be an answer for 'flower' (I've seen this before). I am not sure about the 'flatter' bit.
'merest trace of canker in' is the wordplay.
'merest' becomes 'butterup' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'trace of' says to take the initial letters.
'in' indicates putting letters inside.
The first letter of 'canker' is 'c'.
'butterup' going around 'c' is 'BUTTERCUP'.
Can you help me to learn more?
(Other definitions for buttercup that I've seen before include "Yellow-flowered grassland plant" , "Rich yellow wild flower" , "growth in the field" , "found in meadow" , "Flowering plant" .)