Determined to get the underworld crooked (8)
I believe the answer is:
hellbent
'determined' is the definition.
(I've seen this before)
'get the underworld crooked' is the wordplay.
'get' means one lot of letters go next to another.
'the underworld' becomes 'hell' (synonyms).
'crooked' becomes 'bent' (similar in meaning).
'hell' next to 'bent' is 'HELLBENT'.
'to' is the link.
(Other definitions for hellbent that I've seen before include "Recklessly determined - on way to Hades?" , "Determined to achieve something at all costs" , "Utterly determined on road to perdition?" , "Implacably set (on)" , "Utterly determined, on way to perdition" .)