Hi N - could be a mis-written English version of Llaneilfyw (English is: 'St. Elvis' - honestly - really is!) if its a birth certificate you have to refer to - English naming for Welsh, Scottish, Irish and other regional place names is fairly common especially if the name was spoken rather than given in writing...
Llaneilfyw is in Pembrokeshire.
Boring things you learn as you go through life but it might help your quest!- 'LLan' is a Welsh name place for a field or enclosure- usually after a saints name. As Obi Wan says, 'Find St Cloves, Luke, find the St Cloves.....' and you might find the location...
And by the way the Catholic Forum list of saints doesn't have a St Cloves - nearest name I guess is St. Cloud - founded a town in France called Saint Cloud, near Paris...so lokks like you do want St Clears after all in Wales...