Expensive beginning for a letter (4)
I believe the answer is:
dear
'expensive' is the definition.
(similar in meaning)
'beginning for a letter' is the wordplay.
'beginning for' becomes 'de' (I can't explain this - if you can you should believe this answer much more).
'letter' becomes 'r'.
'de'+'a'+'r'='DEAR'
Can you help me to learn more?
(Other definitions for dear that I've seen before include "Honey - costly" , "It's not cheap" , "Beloved or costly" , "Expensive; sweet" , "Costly (but loved!)" .)