Initially complex indeed, penning woman’s secret message (6)
I believe the answer is:
cipher
'secret message' is the definition.
(message in code)
'initially complex indeed penning woman's' is the wordplay.
'initially' suggests taking the first letters.
'woman's' becomes 'her' (belonging to a woman).
The first letters of 'complex indeed penning' is 'cip'.
'cip'+'her'='CIPHER'
(Other definitions for cipher that I've seen before include "A message written in code" , "Symbol representing nought, thing of no importance" , "Key to a code" , "Secret code" , "Code - 0" .)