The term 'half a semibreve' refers to a musical note that is half the length of a semibreve. A semibreve is the longest note in modern musical notation and is equal to two beats in 4/4 time. The crossword solution for this term is 'MINIM, ' which is a musical note that is half the length of a crotchet (or quarter note). Therefore, a MINIM is equal to one beat in 4/4 time and is half the length of a semibreve.