Careless treatment of linnet egg (9)
I believe the answer is:
negligent
'careless' is the definition.
(similar in meaning)
'treatment of linnet egg' is the wordplay.
'treatment of' indicates anagramming the letters (letters treated or processed).
'linnet'+'egg'='linnetegg'
'linnetegg' anagrammed gives 'NEGLIGENT'.
(Other definitions for negligent that I've seen before include "sloppy" , "lacking care" , "Lax" , "slack" , "Failing to take due care" .)