At great cost, he's taken in by the mad hatter (3,5)
I believe the answer is:
the earth
'at great cost' is the definition.
(it cost the earth means at great cost)
'he's taken in by the mad hatter' is the wordplay.
'taken in by' indicates putting letters inside.
'the mad' indicates anagramming the letters.
'hatter' is an anagram of 'tearth'.
'he' going into 'tearth' is 'THE EARTH'.
Want a hint initially instead of a full solution?
Install my app