Suppose unity encompasses mathematical ratio (5)
I believe the answer is:
opine
'suppose' is the definition.
(I've seen this before)
'unity encompasses mathematical ratio' is the wordplay.
'unity' becomes 'one' (synonyms).
'encompasses' means one lot of letters goes inside another (some letters encompass or go around others).
'mathematical ratio' becomes 'pi' (the number pi).
'one' enclosing 'pi' is 'OPINE'.
(Other definitions for opine that I've seen before include "Think - express a view" , "Give opinion" , "Consider" , "Suggest" , "Express a point of view" .)