Bachelor, one in post with English sweetheart (9)
I believe the answer is:
columbine
'sweetheart' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'bachelor one in post with english' is the wordplay.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'post with' becomes 'column' (I've seen this before. I am not sure about the 'with' bit.).
'english' becomes 'E' (abbreviation).
'b'+'i'='bi'
'bi' placed into 'column' is 'columbin'.
'columbin'+'e'='COLUMBINE'
Can you help me to learn more?
(Other definitions for columbine that I've seen before include "Blue coin M. gives for a flowering plant" , "Garden flower - Harlequin's partner" , "One with Harlequin, Pierrot, etc" , "girl in pantomime" , "servant girl in old panto" .)