Get together against backing a dog and a horse (4,2)
I believe the answer is:
gang up
'get together against' is the definition.
I can't judge whether this definition defines the answer.
'backing a dog and a horse' is the wordplay.
'backing' shows that the letters should be reversed in order.
'a dog' becomes 'pug' (pug is a kind of dog).
'and' means one lot of letters go next to another.
'a horse' becomes 'nag' (term for an old horse).
'pug'+'nag'='pugnag'
'pugnag' back-to-front is 'GANG UP'.
Can you help me to learn more?
(Another definition for gang up that I've seen is " Act as an organized group against someone".)