Monstrous hound, far roaming, brought about Baskerville’s end (7,2)
I believe the answer is:
Here is my best explanation:
'monstrous' is the definition.
'hound far roaming brought about baskerville's end' is the subsidiary indication.
'roaming' indicates an anagram.
'brought about' indicates putting letters inside.
'end' indicates one should take the final letters.
The last letter of 'baskerville' is 'e'.
'houndfar' with letters rearranged gives 'unhardof'.
'unhardof' placed around 'e' is 'unheardof'.
(Other definitions for unheard-of that I've seen before include "Fear hound being not previously known", "Without precedent", "Not previously known or done - inaudible?", "Unknown - novel" and "Previously unknown".)