Chooses to include single Conservative in scientific study (6)
I believe the answer is:
optics
'scientific study' is the definition.
(I've seen this before)
'chooses to include single conservative' is the wordplay.
'chooses' becomes 'opts' (I've seen this in another clue).
'to include' means one lot of letters goes inside another.
'single' becomes 'I' (one).
'conservative' becomes 'C' (UK political abbreviation).
'i'+'c'='ic'
'opts' going around 'ic' is 'OPTICS'.
'in' acts as a link.
(Other definitions for optics that I've seen before include "Drink measuring devices" , "The scientific study of light and vision" , "Organs of sight, eyes" , "Study of the physical properties of light" , "Science of light/vision" .)