Thin pastry split on the outside by uncultured learner (7)
I believe the answer is:
strudel
'thin pastry' is the definition.
'split on the outside by uncultured learner' is the wordplay.
'on the outside' says to hollow out the word (remove centre letters).
'by' means one lot of letters go next to another.
'uncultured' becomes 'rude'.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'split' with its middle taken out is 'st'.
'st'+'rude'+'l'='STRUDEL'
(Other definitions for strudel that I've seen before include "Germany pastry" , "German dessert" , "German fruit-filled pastry" , "A German baked pastry" , "food" .)