Official document to be completed by a learner driver (6)
I believe the answer is:
formal
'official' is the definition.
(similar in meaning)
'document to be completed by a learner driver' is the wordplay.
'document' becomes 'form' (form is a kind of document).
'to be completed by' says to put letters next to each other (some letters go at the end of others).
'learner driver' becomes 'l' (learner drivers have L-plates on their cars).
'form'+'a'+'l'='FORMAL'
(Other definitions for formal that I've seen before include "Punctilious" , "Conforming to accepted rules, as of dress" , "Stiff - proper" , "It's required by rule" , "Precise - ceremonial" .)