Silver-tongued model took posh men around sierra (6-6)
I believe the answer is:
smooth-spoken
'silver-tongued' is the definition.
(synonyms)
'model took posh men around sierra' is the wordplay.
'model' indicates anagramming the letters.
'around' indicates putting letters inside.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'took'+'posh'+'men'='tookposhmen'
'tookposhmen' with letters rearranged gives 'smoothpoken'.
'smoothpoken' placed around 's' is 'SMOOTH-SPOKEN'.
(Another definition for smooth-spoken that I've seen is " Plausible".)