Learn Icelandic: Klettaklifur er mjög öfgafull íþrótt. - Rock climbing is a very extreme sport.

In Icelandic compound nouns, the first part is often placed in the genitive (possessive) case. Kletta is the genitive plural of klettur (rock/cliff). It is in the plural because rock climbing generally involves scaling multiple rocks, making it literally the "climbing of rocks."

Why is the adjective öfgafull in the feminine form when the subject, klettaklifur, is a neuter noun?

The adjective is directly modifying the predicate noun íþrótt (sport), which is feminine. Because it functions as an attributive adjective inside the noun phrase mjög öfgafull íþrótt, it must agree with íþrótt in gender, number, and case (feminine, singular, nominative).

What are the component parts of the adjective öfgafull?

It is formed from the feminine plural noun öfgar (extremes, excesses) and the suffix -fullur (full of). Therefore, an "extreme" sport is quite literally described as a sport "full of extremes" in Icelandic.

Could another intensifier be used instead of mjög in this context?

Definitely. While mjög is the most standard word for "very", you could use afar (extremely) for a slightly more formal or written tone. In colloquial speech, Icelanders often use words like gríðarlega (hugely) or svakalega (insanely/terribly) to add dramatic emphasis when talking about extreme sports.