The whole sentence is "Few things are impossible in themselves; and it is often for want of will, rather than of means, that man fails to succeed". It's a quote from Francois de La Rochefoucauld.
The "that" in " ..., that man fails to succeed" made me a bit confused. From my point of view, the latter part of the sentence is to say "man fails to succeed due to the lacking of want of will". Thus the "that" here sounds more like "thus" or "in this way". But looking up in the dictionary, I didn't find such a meaning of "that". So what is "that" mean here and what is its grammar function in this sentence?