Sixties designer adopting a doctor's measuring device (8)
I believe the answer is:
quadrant
'measuring device' is the definition.
(I've seen this before)
'sixties designer adopting a doctor's' is the wordplay.
'sixties designer' becomes 'quant' (I can't explain this - if you can you should give a lot more credence to this answer).
'adopting' is an insertion indicator (inserted letters are taken on or adopted).
'doctor' becomes 'Dr' (abbreviation).
'a'+'dr'='adr'
'quant' enclosing 'adr' is 'QUADRANT'.
Can you help me to learn more?
(Other definitions for quadrant that I've seen before include "measuring device" , "Half of a semicircle" , "Fourth part of circumference" , "One fourth of circle" , "Quarter sector of circle" .)