The most common and most likely answer to this crossword clue is the 7 letter word TAILEND
T
A
I
L
E
N
D
Buddy explains!

The straight part
We know that "Back" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case back = tailend.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TAILEND mean?
TAILEND noun- The hindmost part of anything (a person, animal, or object), the butt, buttocks; hindquarters, rump.
- The last part of a period of time, event, or situation; the concluding or final part.