I trade extremely lucratively — in theory (7)
I believe the answer is:
ideally
'in theory' is the definition.
(thesaurus)
'i trade extremely lucratively' is the wordplay.
'trade' becomes 'deal' ('deal' can be a synonym of 'trade').
'extremely' means to remove the middle letters.
'lucratively' with its centre removed is 'ly'.
'i'+'deal'+'ly'='IDEALLY'
(Other definitions for ideally that I've seen before include "In the most perfect way" , "Lay idle (anag)" , "In a perfect world, the best possible situation" , "In the best of all possible worlds" , "In the best possible way or circumstances" .)