The most common and most likely answer to this crossword clue is the 7 letter word IMMENSE
I've seen this crossword clue in Independent
I
M
M
E
N
S
E
More clues leading to the result IMMENSE
- Very big chaps breaking into dilapidated semi
- Tremendous chaps in fantastic Times putting out leader
- Huge – a tenth of a centimetre seen differently
- Jumbo the setter’s set mostly to trap people
- Enormous Frenchmen plunged into tempestuous Seine
- Huge mines exploding around me
- Hoskins is who I am! One ghastly twerp essentially without limits!
- Giant that is guarding thousand people, foremost in story
- Tremendous, when single chaps embrace married state’s limits
- Extinct at last, my nemesis, woolly mammoth
Buddy explains!

The straight part
We know that "mammoth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mammoth = immense.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 IMMENSE mean?
IMMENSE noun- Immense extent or expanse; immensity
- Huge, gigantic, very large.
- Supremely good.