Blonde careless entering marsh (6)
I believe the answer is:
flaxen
'blonde' is the definition.
(similar in meaning)
'careless entering marsh' is the wordplay.
'careless' becomes 'lax' (synonyms).
'entering' means one lot of letters goes inside another.
'marsh' becomes 'fen' (synonyms).
'lax' put into 'fen' is 'FLAXEN'.
(Other definitions for flaxen that I've seen before include "Light yellow" , "Pale yellow, as hair may be" , "Pale yellowish brown" , "Blonde, reading poetry" , "Pale yellow in colour" .)