Meow and start to enjoy cat milk, regularly put out: it’s placed by front door (7,3)

'it's placed by front door' is the definition.

'meow and start to enjoy cat milk regularly put out' is the wordplay.
'and' means one lot of letters go next to another.
'start to' says to take the initial letters.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'put out' indicates an anagram.
The first letter of 'enjoy' is 'e'.
The alternate letters of 'milk' are 'ml'.
'e'+'cat'+'ml'='ecatml'
'meow' put after 'ecatml' is 'ecatmlmeow'.
'ecatmlmeow' anagrammed gives 'WELCOME MAT'.