'At' as a Preposition is a word that joins a noun, pronoun, or noun phrases and makes each sentence complete. It is used in a sentence to indicate a location, direction, time, describe an activity, or sometimes, to introduce an object.

In this sentence, it is used to indicate Chike’s activity (at dribbling). Hence the complete statement would read;

Chike, who plays in the first eleven, is an expert at dribbling