I am working in a consultancy. We keep getting 2-3 liner job descriptions from our clients. They assume that we would understand the requirement from that. However, when we speak to candidates about it, the candidates request for a detailed description. The companies never send us anything more than that.

In such a case how can we get a more detailed job-description?

Get a resume of the same profile from the same company and the work that the particular candidate does can be used as a JD for other candidates.....

go through CVs of candidate in the same position of that company

If you are an experienced recruiter you should be in a position to prepare the required JD

IM TOTALLY AGREE WITH MR YOUNUS.

When you are lining up a particular candidate then prepare the job description beforehand.
You can possibly ask the job desription to a person who has worked with the same organisation or is currently working in the organisation.