Most of book is ultimately irrelevant for backup soldier (9)
I believe the answer is:
reservist
'backup soldier' is the definition.
'reservist' can be an answer for 'soldier' (reservist is a kind of soldier). I am not certain of the 'backup' bit.
'most of book is ultimately irrelevant' is the wordplay.
'most of' means to remove the last letter.
'book' becomes 'reserve' (as in booking or reserving in advance).
'ultimately' indicates one should take the final letters.
The last letter of 'irrelevant' is 't'.
'reserve' with its last letter taken away is 'reserv'.
'reserv'+'is'+'t'='RESERVIST'
'for' is the link.
Can you help me to learn more?
(Other definitions for reservist that I've seen before include "One liable for call-up" , "militia member" , "Soldier able to be called up" , "one of 51 [THETA] ?" , "Eg, a TA member" .)