A commonplace to prohibit a learner (5)
I believe the answer is:
banal
'a commonplace' is the definition.
(I know that commonplace can be written as banal)
'prohibit a learner' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'ban'+'a'+'l'='BANAL'
'to' is the link.
(Other definitions for banal that I've seen before include "boringly dull" , "Overfamiliar through overuse" , "Ordinary and uninteresting" , "Old hat" , "Uninteresting, commonplace" .)