CLIC will need of order 1014 positrons per second to achieve its specified luminosity [1]. An undulator based scheme has been proposed as one of the options for the positron source to meet this challenge. As CLIC may operate over a wide range of energy (from 0.5 TeV to 3 TeV centre of mass), there is a large scope to push the performance of the whole system to reach high efficiency. We report on the undulator parameters and optimisation of components of the source, focusing on the undulator, and the adiabatic matching device. In addition to maximising the positron yield, the polarisation of the positron beam is also considered.