The phrase "want of substance" refers to the lack of depth, significance, or strong foundation in something. There are various synonyms that can be used to describe this concept including shallowness, superficiality, emptiness, inadequacy, hollowness, insignificance, and insubstantiality. Another way to express the idea could be by using words such as triviality, frivolity, or inconsequence. The term can be applied to different areas, such as art, literature, or even personal relationships. It implies that something lacks a certain level of meaning or relevance, and can be used to criticize or dismiss someone or something as being lightweight or unimportant.