The most common and most likely answer to this crossword clue is the 2 plus 7 letter word IN GENERAL
I've seen this crossword clue in The Times
I
N
G
E
N
E
R
A
L
More clues leading to the result IN GENERAL
- E-learning worked, mainly
- Mostly having drunk gin, with bad renal failure ultimately
- Where army should have faith, as a rule?
- Largely popular officer
- E-learning astray, by and large.
- E-learning is customised mostly
- Novel learning covering English mostly
- Usually home ahead of army officer
- Popular officer, more often than not
- Usually popular army officer
Buddy explains!

The straight part
We know that "mainly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mainly = in general.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.