Eleventh-century families in Normandy were only just beginning to use place-names to identify themselves. At the same time they were using the Norman fitz to prefix a name to signify 'the son of'. Hence Robert Fitz-Hamon was Hamon's son, Robert, but with no additional reference to a place-name to identify him.
If they had been consistent, all of Mauger's sons and grandsons could have called themselves 'de St Clair', unless of course they had been granted titles based on lands elsewhere. Clearly this did not happen because only Walderne's sons were referred to as 'de Sancto Claro' (the latin version of 'de St Clair') in Domesday in 1086.
The use of 'de St Clair' in contemporary documents from 1120 suggests strongly that those carrying this name were the direct sons or grandsons of Walderne. Although it is unlikely that this can ever be confirmed, the alternative proposition to name all of Mauger's descendents as 'de St Clair' runs the risk of superimposing twentieth-century customs on eleventh- and twelfth-century England, and thereby claiming for Sinclair family ancestry far more than would otherwise be justified.