I thought clun dost was ear ache not sore hip?
Clust dost=Ear ache, Clun dost=Hip ache.
Why gwddw tost but clun dost?
Because "Gwddw" is masculine and "Clun" is feminine, and adjectives mutate softly after singular feminine nouns.
Why ofnadwy clun o tost but clun tost?
Clun dost = a sore/bad hip
Clun dost ofnadwy/Clun ofnadwy o dost (both word orders are possible) = a terribly sore/bad hip
soft mutation after 'o'