"Not entirely" is a phrase that usually implies a degree of uncertainty or incompleteness. There are several synonyms that can be used to replace this phrase, such as "partially," "somewhat," "incompletely," "imperfectly," "partly," "to a certain extent," "not wholly," "not fully," and "not completely." Each of these synonyms conveys a slightly different nuance, depending on the context of the sentence. For instance, "partially" suggests a portion of something is missing or incomplete, while "somewhat" implies a moderate level of uncertainty or lack of completeness. These synonyms can be useful in making writing more concise or in adding variety to one's vocabulary.