Online magazine with a "Dear Prudence" advice column (5)
I believe the answer is:
slate
'online magazine with a dear prudence advice column' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slate that I've seen before include "Roof tile or pub credit" , "Blast" , "Rubbish" , "Metamorphic rock that can be split into thin layers" , "Roofing material or credit recorder" .)