Sincere but crude (7)
I believe the answer is:
artless
This clue is a double definition.
'sincere' is the first definition.
(associated in meaning)
'crude' is the second definition.
(synonyms)
'but' is the link.
(Other definitions for artless that I've seen before include "Without guile or affectation" , "Simple, unaffected" , "Without pretension" , "as an empty gallery" , "Nave" .)