Strict rule, finally, so half-decent vet won't do this to cat (6)

'half-decent vet won't do this to cat' is the definition.
The definition suggests a singular noun but the answer is not.

'strict rule finally' is the wordplay.
'strict' becomes 'dec' (I can't justify this - if you can you should give a lot more credence to this answer).
'rule' becomes 'law' (law is a kind of rule).
'finally' is a charade indicator (letters next to each other) (some letters go at the end of others).
'dec'+'law'='DECLAW'

'so' acts as a link.

Can you help me to learn more?