The most common and most likely answer to this crossword clue is the 7 letter word MISSION
I've seen this crossword clue in The Daily Telegraph
M
I
S
S
I
O
N
More clues leading to the result MISSION
- Goal? Fail to score with keeper getting back
- Girl – one on errand
- Undertaking not entirely lost without love
- Girl one takes on an assignment
- Neglect one on purpose
- Young lady I will take on assignment
- Fail to catch a particle on this specific flight
- Girl, one concerned with exercise
- Calling woman from the South numero uno
- Don't hit one over goal
Buddy explains!

The straight part
We know that "assignment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assignment = mission.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 MISSION mean?
MISSION noun- A set of tasks that fulfills a purpose or duty; an assignment set by an employer, or by oneself.
- Religious evangelism.
- (in the plural, "the missions") third world charities, particularly those which preach as well as provide aid.
- (Catholic tradition) an infrequent gathering of religious believers in a parish, usually part of a larger regional event with a central theme.
- A number of people appointed to perform any service; a delegation; an embassy.
- Dismissal; discharge from service
- A settlement or building serving as a base for missionary work.
- To send on a mission.
- Do missionary work, proselytize