Greek character pinned by relatively insignificant monster … (8)
I believe the answer is:
minotaur
'monster' is the definition.
(Minotaur is an example)
'greek character pinned by relatively insignificant' is the wordplay.
'greek character' becomes 'tau' (letter from the Greek alphabet).
'pinned by' indicates putting letters inside.
'relatively insignificant' becomes 'minor'.
'tau' going within 'minor' is 'MINOTAUR'.
(Other definitions for minotaur that I've seen before include "Mythical creature" , "Creature, half-man and half-bull, slain by Theseus in the Cretan maze" , "Mythical man-bull" , "Cretan labyrinth resident" , "Mythical beast of Crete" .)