Child captures cross subterranean beast (8)
I believe the answer is:
minotaur
'subterranean beast' is the definition.
'minotaur' can be an answer for 'beast' (I have seen 'Beast of legend' mean 'minotaur' so perhaps 'beast' could also mean 'minotaur'). I'm not certain of the 'subterranean' bit.
'child captures cross' is the wordplay.
'child' becomes 'minor' ('minor' can be a synonym of 'child').
'captures' is an insertion indicator.
'cross' becomes 'tau' (tau cross is a kind of cross**).
'minor' going around 'tau' is 'MINOTAUR'.
Can you help me to learn more?
(Other definitions for minotaur that I've seen before include "Mythical beast of Crete" , "50 per cent ruminant" , "awful beast" , "Mythical creature" , "Mythological man-bull" .)