Note that the top has been simplified and is now graphically 士 (the left vertical mark has migrated up the main vertical, becoming horizontal). This simplified form is found in seal characters, where it coexisted with forms where the top was more recognizably 止.
This is one of only three everyday kanji with a 5 morae reading, and one of only two nouns, namely 志 (こころざし, kokorozashi) (nominalization of form of verb 志す (こころざす, kokorozasu), which has a four-mora reading). The only other kanji with five-mora readings are 承, in the verb 承る (（うけたまわ）る, uketamawa-ru), and 詔 (みことのり, mikotonori). No kanji in common use has a six- or more mora reading.
The unusually long reading is due to two factors: 志 is a nominalization of the verb 志す which has a long reading (4 morae, kokoroza-su), which itself is a noun-verb compound, 心指す kokoro-za(su). The nominalization removing the okurigana, hence increasing the reading by one morae, yielding 4+1=5, or more finely 3+(1+1)=5 (compare common 話 (はなし, hanashi) 2+1=3, from 話す (はなす, hana-su)).