Dose working with pet at first, before it's put down (7)
I believe the answer is:
deposit
'put down' is the definition.
(I've seen this before)
'dose working with pet at first before it's' is the wordplay.
'working with' indicates anagramming the letters (working with the letters).
'at first' says to take the initial letters.
'before' means one lot of letters go next to another.
'it's' becomes 'it' (I've seen this in other clues**).
The first letter of 'pet' is 'p'.
'dose' with its letters mixed up with 'p' can make 'depos'.
'depos'+'it'='DEPOSIT'
(Other definitions for deposit that I've seen before include "Put money into a bank account" , "Put down or leave something" , "Drop" , "is one back on the bottle?" , "Part payment in advance to secure goods" .)