Vouch is a term that means to support or endorse something. It is often used in the context of providing assurance or backing for someone or something. In this case, the phrase 'Speak Up (For)' can be interpreted as an encouragement to speak up and provide support for a cause or person. Therefore, the crossword solution of VOUCH for the term 'Speak Up (For)' implies that one should use their voice to advocate for something they believe in.