The entire world is looking to fulfill the need of the hour in terms of security. Certificateless cryptography is an efficient approach studied widely due to two reasons: first, it eliminates the need of certificate authority in public key infrastructure and second, it can resolve key escrow problem of ID-based cryptography. Recently, Zhang et al. proposed a novel security scheme based on RSA, applicable to real life applications but could not cope up with the well defined attacks. This paper, presents an RSA-based CertificateLess Signature (RSA-CLS) scheme applicable to wireless sensor networks. The security of RSA-CLS is based on the hardness assumption of Strong RSA. The scheme is proven to be secure against Type I and Type II attack in random oracle model.