Ailing king departs thus (3)
I believe the answer is:
sic
'thus' is the definition.
('thus' in Latin)
'ailing king departs' is the wordplay.
'ailing' becomes 'sick' (similar in meaning).
'king' becomes 'K' (chess abbreviation).
'departs' indicates named letters should be taken away.
'sick' with 'k' removed is 'SIC'.
(Other definitions for sic that I've seen before include "Urge to attack" , "'Thus, in printing (3)'" , "Intentionally so written" , "(Of quoted word) exactly so" , "Like" .)