Mint with pretty flowers seems to develop last of all (6)
I believe the answer is:
hyssop
'mint' is the definition.
(plant of the mint family)
'with pretty flowers seems to develop last of all' is the wordplay.
'last of all' says to take the final letters.
The final letters of 'with pretty flowers seems to develop' are 'HYSSOP'.
(Other definitions for hyssop that I've seen before include "Plant with minty leaves used in cooking or as medicine" , "little blue flowers" , "Aromatic herb" , "Small bushy plant with bitter minty leaves" , "Aromatic plant" .)