It's correct to be constant (4)
I believe the answer is:
true
I believe this is a double definition.
'it's correct' is the first definition.
(associated in meaning)
'be constant' is the second definition.
(similar in meaning)
'to' is the link.
(Other definitions for true that I've seen before include "Genuine; faithful" , "Accurate, factual" , "Real, actual" , "Agreeing with fact or reality" , "Straight or loyal" .)