Truth is basically reassuring, using logic’s essential (4)
I believe the answer is:
rule
'truth' is the definition.
(thesaurus)
'basically reassuring using logic's essential' is the wordplay.
'basically' suggests taking the first letters (the first letter could be its root or base).
The initial letters of 'reassuring using logics essential' is 'RULE'.
'is' is the link.
(Other definitions for rule that I've seen before include "Dominion" , "Exercise authority over" , "Strip" , "Dominate" , "Govern, reign over" .)