Does anyone know where I can get a Monolingual Welsh dictionary
The closest thing to a monolingual Welsh dictionary that I am aware of is “Geiriadur Prifysgol Cymru”, available in book form in four volumes (and also as an app). It lists Welsh words alphabetically and gives the definition in Welsh. It also gives english equivalents in the entries, but no definition in english, and no english section ordered alphabetically.
Edit to add: Finding the actual printed books may be hard and/or costly, as they are out-of-print. The app version is free, though 
Thanks Hendrik. That’s helpful.
I have such a dictionary and when its kight will search for it Its heavy book I purchased from the shop Y Siswn in Mold N Wales. Written for welsh readers. Its goodAlso you may want to download an internet app called PDC. there is an English option in it but the Welsh version I find pretty good details as below, and i will let you know name if hard cover options
Geiriadur Prifysgol Cymru Hawlfraint © Prifysgol Cymru Y Drindod Dewi Sant, 2020
Cedwir pob hawl. Ni cheir atgynhyrchu unrhyw ran o’r cyhoeddiad hwn na’i gadw mewn cyfundrefn adferadwy na’i drosglwyddo mewn unrhyw ddull na thrwy unrhyw gyfrwng electronig, mecanyddol, ffotocopïo, recordio, nac fel arall, at ddibenion masnachol heb ganiatâd ymlaen llaw gan Geiriadur Prifysgol Cymru, Y Ganolfan Uwchefrydiau Cymreig a Cheltaidd, Llyfrgell Genedlaethol Cymru, Aberystwyth, Ceredigion, SY23 3HH.
Rhif cofrestredig y cwmni: RC000537 (Cymru a Lloegr)
Rhif elusen: 1149535 hi’n