What does the cryptic crossword clue "Tall crop in Africa - a spear’s length potentially (8,5)" mean?

The wordplay uses an anagram. anagram of A + SPEAR’S + LENGTH = ELEPHANTGRASS The answer is ELEPHANTGRASS.

Tall crop in Africa - a spear’s length potentially (8,5)

Daily Mail Cryptic #16539 | 19 Across | 2021-03-24 flower