Do to keep one's equivalence (6)
I believe the answer is:
parity
'equivalence' is the definition.
(I know that parity is a type of equivalence)
'do to keep one's' is the wordplay.
'do' becomes 'party' (I've seen this before).
'to keep' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'party' going around 'i' is 'PARITY'.
(Other definitions for parity that I've seen before include "Equal condition" , "equal treatment" , "Egalitarian goal" , "Correspondence" , "Equality, equivalence" .)