Monster controlling the middle (5)
I believe the answer is:
troll
'monster' is the definition.
(I know that troll is a type of mythical monster)
'controlling the middle' is the wordplay.
'the middle' means to look at the middle letters.
The centre of 'controlling' is 'TROLL'.
(Other definitions for troll that I've seen before include "supposedly lives in a cave" , "Fish by trailing a baited line" , "Unsocial social media user" , "Scandinavian dwarf or giant" , "Monster - fish" .)