Genuinely attempt to encompass central rule (5)
I believe the answer is:
truly
'genuinely' is the definition.
(I've seen this in another clue)
'attempt to encompass central rule' is the wordplay.
'attempt' becomes 'try' (synonyms).
'to encompass' is an insertion indicator (I've seen this in other clues).
'central' says to take the centre.
The central letters of 'rule' are 'ul'.
'try' enclosing 'ul' is 'TRULY'.
(Other definitions for truly that I've seen before include "Genuinely, properly" , "Indeed" , "Honestly" , "with accuracy" , "Really, genuinely" .)