With law on union rule, soldier makes a complaint (5)
I believe the answer is:
lurgi
'a complaint' is the definition.
(I have seen 'Funny complaint' mean 'lurgi' so perhaps 'complaint' could also mean 'lurgi')
'with law on union rule soldier' is the wordplay.
'with' means one lot of letters go next to another.
'law on union rule' becomes 'lur' (I can't justify this - if you can you should believe this answer much more).
'soldier' becomes 'GI' (term for US soldier).
'lur'+'gi' is 'LURGI'.
'makes' is the link.
Can you help me to learn more?
(Other definitions for lurgi that I've seen before include "Unspecified illness" , "Non-specific disease" , "Funny complaint" , "disease not taken seriously" , "Invented disease" .)