Rock legend's first principle (8)
I believe the answer is:
keystone
'principle' is the definition.
(thesaurus)
'rock legend's first' is the wordplay.
'rock' becomes 'stone' (synonyms).
'legend' becomes 'key' (legend can mean a key on a map or diagram).
'first' is a charade indicator (letters next to each other).
'stone' after 'key' is 'KEYSTONE'.
(Other definitions for keystone that I've seen before include "Piece at the top of a masonry arch" , "Chief element" , "What tops an arch" , "everything may depend on it" , "Vital support" .)