Answer
The adjective of the word 'Leader' is 'Lead'.
Explanation :
The word "lead" is a verb, a noun, and an adjective.
Examples:
Jim was the leader of the expedition. (noun)
He lead his three companions through the mall. (verb)
Jim took the lead in their quest for sustenance. (noun)
He was the lead searcher (adjective)
The noun "lead" is also a word for a metallic element.