Soldier to go off, it's absolute nonsense (8)
I believe the answer is:
tommyrot
'absolute nonsense' is the definition.
(I've seen this before)
'soldier to go off' is the wordplay.
'soldier' becomes 'tommy' (I have seen 'British soldier' mean 'tommy' so perhaps 'soldier' could also mean 'tommy').
'to go off' becomes 'rot' (I've seen this before).
'tommy'+'rot'='TOMMYROT'
'it's' is the link.
(Other definitions for tommyrot that I've seen before include "Absolute rubbish, nonsense" , "tosh" , "Poppycock" , "Bunk" , "Nonsense, rubbish - ask informal Thomas" .)