Motto, for example, popular with Guides, primarily (6)
I believe the answer is:
saying
'motto' is the definition.
(thesaurus)
'for example popular with guides primarily' is the wordplay.
'for example' becomes 'say' (I've seen this before).
'popular' becomes 'in'.
'with' is a charade indicator (letters next to each other).
'primarily' indicates taking the first letters.
The first letter of 'guides' is 'g'.
'say'+'in'+'g'='SAYING'
(Other definitions for saying that I've seen before include "Adage, maxim" , "Saw" , "Speaking; adage" , "Expressing in words" , "Gnome" .)