Wicked larcenist losing 'is key (7)
I believe the answer is:
central
'key' is the definition.
('central' can be a synonym of 'key')
'wicked larcenist losing 'is' is the wordplay.
'wicked' is an anagram indicator.
'losing' indicates named letters should be taken away.
'larcenist' with 'is' taken out is 'larcent'.
'larcent' with letters rearranged gives 'CENTRAL'.
(Other definitions for central that I've seen before include "Middle; crucial" , "Key" , "Most important, principal" , "Every answer here has one such letter" , "Near the middle" .)