Ailsa, run at new Roman festival? (10)
I believe the answer is:
saturnalia
'roman festival?' is the definition.
(I've seen this before)
'ailsa run at new' is the wordplay.
'new' indicates anagramming the letters.
'ailsa'+'run'+'at'='ailsarunat'
'ailsarunat' anagrammed gives 'SATURNALIA'.
(Other definitions for saturnalia that I've seen before include "orgy" , "Ancient Roman December festival" , "Wild Roman festival" , "Old festival" , "wild party" .)