Introduction of conscription, soon to become law (5)
I believe the answer is:
canon
'law' is the definition.
(thesaurus)
'introduction of conscription soon' is the wordplay.
'introduction of' says to take the initial letters.
'soon' becomes 'anon' (I've seen this before).
The initial letter of 'conscription' is 'c'.
'c'+'anon'='CANON'
'to become' is the link.
(Other definitions for canon that I've seen before include "accepted criterion" , "priest for cathedral duties" , "Body of established rules or principles" , "Moron" , "Rule -- principle" .)