Travel to and fro with one out to offer friendly greeting (4,9)
I believe the answer is:
good afternoon
'friendly greeting' is the definition.
('good afternoon' is a greeting)
'travel to and fro with one out' is the wordplay.
'travel' becomes 'go' (synonyms).
'with' says to put letters next to each other.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'to'+'and'+'fro'+'one'='toandfroone'
'toandfroone' anagrammed gives 'odafternoon'.
'go'+'odafternoon'='GOOD AFTERNOON'
'to offer' acts as a link.
(Other definitions for good afternoon that I've seen before include "PM greeting" , "Words of salutation" .)