Frankly, a German writer has penned a word in French after reflection (3-2-3)
I believe the answer is:
man-to-man
'frankly' is the definition.
(I've seen this before)
'a german writer has penned a word in french after reflection' is the wordplay.
'a german writer' becomes 'mann' (Thomas Mann, 1875-1955).
'has penned' means one lot of letters goes inside another (inserted letters are enclosed or penned in).
'word in french' becomes 'mot' (I've seen this in another clue).
'after reflection' is a reversal indicator.
'a'+'mot'='amot'
'amot' back-to-front is 'toma'.
'mann' going around 'toma' is 'MAN-TO-MAN'.
(Other definitions for man-to-man that I've seen before include "With frankness, as father may speak to son" , "Directly and frankly" , "Frankly and confidentially" , "Frank and confidential" , "Between males, with frankness" .)