The term 'Hecklephone Relatives' refers to a family of woodwind instruments related to the Heckelphone, which is a double-reed instrument similar to an oboe. The crossword solution for this term is 'oboes, ' which are also double-reed instruments that produce a sound similar to the Heckelphone. Oboes are commonly used in orchestras and other musical ensembles, and they are known for their distinctive, mellow tone.