The most common and most likely answer to this crossword clue is the 7 letter word EVOLVER
I've seen this crossword clue in Independent
E
V
O
L
V
E
R
More clues leading to the result EVOLVER
Buddy explains!
The straight part
We know that "gun blowing head off (7)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gun blowing head off (7) = evolver.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.