Material in study I am sunk beneath (5)
I believe the answer is:
denim
'material' is the definition.
(type of fabric)
'study i am sunk beneath' is the wordplay.
'study' becomes 'den' (term for a study or office).
'i am' becomes 'I'm' (contraction).
'sunk beneath' is a charade indicator (letters next to each other) (in a down clue, some letters sink below others).
'den'+'im'='DENIM'
'in' is the link.
(Other definitions for denim that I've seen before include "Durable material" , "Hard-wearing cotton fabric" , "Cloth for jeans, etc" , "maker of jeans" , "Twilled cotton fabric" .)