Additional matter involving learner driver (4)
I believe the answer is:
plus
'additional' is the definition.
(thesaurus)
'matter involving learner driver' is the wordplay.
'matter' becomes 'pus' (pus is a kind of matter**).
'involving' is an insertion indicator.
'learner driver' becomes 'l' (I've seen this in other clues).
'pus' enclosing 'l' is 'PLUS'.
(Other definitions for plus that I've seen before include "Added extra" , "being a little cross" , "sign of more to come" , "As well as" , "An advantage (colloq.)" .)