Ailing girl harbours English criminal (7)
I believe the answer is:
illegal
'criminal' is the definition.
(similar in meaning)
'ailing girl harbours english' is the wordplay.
'ailing' becomes 'ill' (similar in meaning).
'girl' becomes 'gal'.
'harbours' means one lot of letters goes inside another (harbour can mean to hide or give shelter to).
'english' becomes 'E' (abbreviation).
'ill'+'gal'='illgal'
'illgal' enclosing 'e' is 'ILLEGAL'.
(Other definitions for illegal that I've seen before include "Such an immigrant" , "Wrong" , "murder sure is!" , "Prohibited by law" , "Not allowed" .)