Strictness with truth, following this rule, ultimately (8)
I believe the answer is:
severity
'strictness' is the definition.
(severity is a kind of strictness)
'truth following this rule ultimately' is the wordplay.
'truth' becomes 'verity' ('verity' can be a synonym of 'truth'**).
'following' says to put letters next to each other.
'ultimately' suggests the final letters.
The final letters of 'this rule' are 'se'.
'verity' after 'se' is 'SEVERITY'.
'with' is the link.
(Other definitions for severity that I've seen before include "Sternness" , "Harshness, asperity" , "Intensity; harshness" .)