kasulikku
Kasulikku is the partitive singular form of the Estonian adjective kasulik, meaning useful or beneficial. In Estonian, adjectives decline to agree with the noun they modify in number and case, and kasulikku appears specifically before a noun in the partitive case. This form is typically used when the noun denotes an indefinite quantity or a non-countable concept, such as information or advice.
Etymology and function: kasulik is formed from the root kasul- (benefit, usefulness) with the suffix -lik, which
Usage notes: Kasulikku frequently appears with abstract or mass nouns in partitive, signaling an unspecified portion
See also: kasulik (dictionary form of the adjective), Estonian grammar, Partitive case.
In summary, kasulikku is a standard inflected form used to express “useful” in front of a noun