Soldier writing most of book first (9)
I believe the answer is:
reservist
'soldier' is the definition.
(I know that reservist is a type of soldier)
'writing most of book first' is the wordplay.
I cannot really understand how this works, but
'writing' could be 'r' (one of the traditional three R's of education) and 'r' is present in the answer.
'first' could be 'ist' (ist resembles 1st) and 'ist' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for reservist that I've seen before include "militia member" , "Back-up soldier" , "Soldier called up when needed" , "Soldier able to be called up" , "Eg, a TA member" .)