There is nothing special about the word clenched: virtually all participles can be used not only in verb phrases alone but also as participial adjectives in noun phrases.
However, you should also keep in mind that not all words that modify nouns are adjectives, nor does modification of a noun automatically make a word an adjective.
In answer to the question posed in comments as to whether in
Lack of infrastructure facility can be a major cause for under development.
The word infrastructure is an adjective or a noun, the answer is that it is
a noun, not an adjective.
If it were an adjective, it would accept adverbs as modifiers, but it does not: you cannot say a ∗very infrastructure development or a ∗quickly infrastructure development.
Because it is a noun, it accepts as modifiers either adjectives or other nouns, as in critical infrastructure development or city infrastructure development.
The adjective corresponding to the noun infrastructure is infrastructural.
Modifying nouns does not make in and of itself make something an adjective; there are other criteria. Nouns can modify nouns, and so can verbs.