They're seldom seen: essentially drab grim items of neckwear (8)
I believe the answer is:
rarities
'they're seldom seen' is the definition.
'essentially drab grim items of neckwear' is the wordplay.
'essentially' indicates the central letters.
'grim' becomes 'ri' (I can't justify this - if you can you should believe this answer much more).
'items of neckwear' becomes 'ties' (I've seen this before).
The central letters of 'drab' are 'ra'.
'ra'+'ri'+'ties'='RARITIES'
Can you help me to learn more?
(Other definitions for rarities that I've seen before include "Those things seldom seen" , "collector's items" , "freaks" , "Scarce and valuable things" , "Curios" .)