He later became fascinated by jazz and the music of Duke Ellington and thus chose to continue his studies at the Royal Conservatory of Liège in saxophone and clarinet.

During a performance with Godwin in 1936, he received a commission to play with the Tonhalle Orchester Zürich, where he remained until 1939, when the threat of World War II became too great.

[citation needed] Cleber came into contact with AVRO, a Dutch public broadcaster, and in 1940, he joined their orchestra as a trombonist under the direction of Elzard Kuhlman.

During this time, he also continued his studies at the Conservatory of the Muzieklyceum Society in conducting, harmony, and counterpoint, receiving lessons from Kees van Baaren.

He continued playing with De Ramblers until 1945, when on the request of conductor Dolf van der Linden, he became a trombonist and arranger for the new Metropole Orkest.

He remained with them until 1948, making several studio recordings with van der Linden, and at the same time, he led Selecta, his own ensemble of twelve musicians.

[citation needed] In the late 1950s and early 1960s, Cleber also worked as a staff conductor for Phonogram, making recordings with Conny Stuart, Corry Brokken, Mieke Telkamp, Jules de Corte, and Willy Alberti, among others.

He remained at AVRO for two years as a music advisor, and from 1968 until his retirement in 1981, he produced the program Jonge mensen op weg naar het concertpodium.