De or het klontering?

The correct Dutch article is:

de klontering

Rule Words ending in -ing: almost always de. See the rule

How to use klontering

the klonteringde klontering
this klonteringdeze klontering
that klonteringdie klontering
our klonteringonze klontering
your klonteringjouw klontering
every klonteringelke klontering
which klontering?welke klontering?
a big klontering / the big klonteringeen grote klontering, de grote klontering

Dutch words ending in klontering

samenklontering

In compound words, the last part decides the article. See the rule