Counsel outsiders to advertise (6)
I believe the answer is:
advise
'counsel' is the definition.
(I know that counsel can be written as advise)
'outsiders to advertise' is the wordplay.
'outsiders to' says to hollow out the word (remove centre letters) (only letters on the outside of the word).
'advertise' with its middle removed is 'ADVISE'.
(Other definitions for advise that I've seen before include "Suggest, recommend" , "'Counsel, recommend (6)'" , "Counsel, suggest" , "Notify" , "Recommend (course of action)" .)