Staircase feature; new model, but not modern (5)
I believe the answer is:
newel
'staircase feature' is the definition.
(I've seen this in another clue)
'new model but not modern' is the wordplay.
'but not' suggests deleting specific letters.
'modern' becomes 'mod' (synonyms).
'model' with 'mod' taken out is 'el'.
'new'+'el'='NEWEL'
(Other definitions for newel that I've seen before include "Post at base of staircase" , "(Of a flight) upright post" , "domestic post" , "End post on stair rail" , "Top/bottom supporting post of a stair-rail" .)