It's not really a typical approach. I can't recall ever having seen it in fact. For this reason, I can't say for sure whether it would be allowed or not, but examiners are generally pretty conservative about such things, so I would expect not.
In the example you provide, it seems that you want to abbreviate the phrase "message data source content". If there is no other source content in the claims, it would be reasonable to simply say "the source content" later, since the only reasonable interpretation would be that it refers to the message data source content. However, it would also be a reasonable approach to refer to "the message data source content", without any abbreviation. Brevity should not come at the cost of clarity, after all.
In the US, you can also use the description to define terms used in the claims. However, this approach is not typically available in other jurisdictions (notably, Europe), so you would want to at least make sure any definitions in the description are in claim-like language such that they can be straightforwardly inserted into the claims by amendment.