Careful amateur introduced to careless drudge (7)
I believe the answer is:
guarded
'careful' is the definition.
(similar in meaning)
'amateur introduced to careless drudge' is the wordplay.
'amateur' becomes 'a'.
'introduced to' is an insertion indicator.
'careless' is an anagram indicator.
'drudge' anagrammed gives 'gurded'.
'a' going into 'gurded' is 'GUARDED'.
(Other definitions for guarded that I've seen before include "Very reluctant to give out information" , "Protected from a drudge" , "Cautious; watched over" , "Cautious - or protected" , "Cautious, having reservations" .)