At root, foolish about a girl (6)
I believe the answer is:
radish
'at root' is the definition.
'radish' can be an answer for 'root' (radish is a kind of root). I'm not certain of the 'at' bit.
'foolish about a girl' is the wordplay.
'foolish' becomes 'rash' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'a girl' becomes 'Di' (short for Diane or Diana).
'rash' going around 'di' is 'RADISH'.
Can you help me to learn more?
(Other definitions for radish that I've seen before include "Sir had this vegetable" , "Bit of salad" , "Small red salad vegetable" , "Hottish salad plant" , "3 down" .)