Would old soldier decay? Nonsense! (5,3)
I believe the answer is:
tommy rot
'nonsense' is the definition.
(I've seen this before)
'would old soldier decay?' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is located in the answer.
'decay?' could be 'rot' (rot can mean to decay) and 'rot' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tommy rot that I've seen before include "nonsense" , "Absolute rubbish" , "Bull" .)