Obscure medical treatment dons refer to in retrospective study (8)
I believe the answer is:
elliptic
'obscure' is the definition.
('elliptic' can be a synonym of 'obscure')
'medical treatment dons refer to in retrospective study' is the wordplay.
'medical treatment' becomes 'pill'.
'dons' is an insertion indicator (don means to wear or put on).
'refer to' becomes 'cite' ('cite' can be a synonym of 'refer to').
'in retrospective study' is a reversal indicator.
'pill' going inside 'cite' is 'citpille'.
'citpille' reversed gives 'ELLIPTIC'.
(Other definitions for elliptic that I've seen before include "Rounded like an egg" , "Egg-shaped" , "Of a conical section" , "circular? Not quite" , "Oval" .)