Always the learner, filling head still (12)
I believe the answer is:
nevertheless
'still' is the definition.
(synonyms)
'always the learner filling head' is the wordplay.
'always' becomes 'ever' (synonyms).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'filling' is an insertion indicator.
'head' becomes 'ness' (ness is a headland).
'ever'+'the'+'l'='everthel'
'everthel' placed within 'ness' is 'NEVERTHELESS'.
(Other definitions for nevertheless that I've seen before include "at any rate" , "In spite of everything" , "Still" , "anyhow" , "Notwithstanding" .)