This is one of only three everyday kanji with a five-mora reading, and one of only two nouns. The only other kanji with five-mora readings are 承, in the verb 承る(uketamawaru), and 詔(mikotonori). No kanji in common use has a six- or more mora reading.
The unusually long reading is due to two factors: 志(kokorozashi) is a nominalization of the verb 志す(kokorozasu), which has a long reading (four morae, kokoroza-su), which itself is a noun-verb compound, 心指す(kokorozasu). The nominalization absorbs the okurigana, hence increasing the reading by one mora, yielding 4+1=5, or more finely 3+(1+1)=5 (compare common 話(hanashi) 2+1=3, from 話す(hanasu)).