Science that chooses to accept single constant (6)
I believe the answer is:
optics
'science' is the definition.
(optics is a kind of science)
'chooses to accept single constant' is the wordplay.
'chooses' becomes 'opts' (I've seen this before).
'to accept' means one lot of letters goes inside another.
'single' becomes 'i' (one).
'constant' becomes 'c' (mathematical abbreviation).
'i'+'c'='ic'
'opts' going around 'ic' is 'OPTICS'.
'that' acts as a link.
(Other definitions for optics that I've seen before include "Science of vision/light" , "Study of the physical properties of light" , "Science of light" , "Spirit measures" , "The scientific study of light and vision" .)