First of letters penned by women concerning seducer (4)
I believe the answer is:
wolf
'seducer' is the definition.
(I've seen this before)
'first of letters penned by women concerning' is the wordplay.
'first of' suggests taking the first letters.
'penned by' is an insertion indicator.
'women' becomes 'w'.
'concerning' becomes 'of'.
The initial letter of 'letters' is 'l'.
'w'+'of'='wof'
'l' placed within 'wof' is 'WOLF'.
(Other definitions for wolf that I've seen before include "Dog-like animal - womaniser" , "character of Prokofiev's music" , "Pack-hunting carnivore" , "philanderer" , "See 2" .)