In the myths a tyrannical sylvan deity (5)
I believe the answer is:
satyr
'sylvan deity' is the definition.
(I've seen this before)
'in the myths a tyrannical' is the wordplay.
'in' indicates a hidden word.
'SATYR' is hidden within 'myths a tyrannical'.
(Other definitions for satyr that I've seen before include "Woodland man-goat god" , "Greek god of the woodlands" , "He dances in the woods" , "Woodland deity, attendant on Bacchus" , "Lecherous god" .)