Understand, principally, that worthiness isn't guaranteed (4)
I believe the answer is:
twig
'understand' is the definition.
(thesaurus)
'principally that worthiness isn't guaranteed' is the wordplay.
'principally' indicates taking the first letters.
The initial letters of 'that worthiness isnt guaranteed' is 'TWIG'.
(Other definitions for twig that I've seen before include "Tiny end of branch" , "Understand; sprig" , "Suddenly catch" , "Finally understand" , "A little stick" .)