Said rule somehow remaining (8)
I believe the answer is:
residual
'remaining' is the definition.
(I know that remaining can be written as residual)
'said rule somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'said'+'rule'='saidrule'
'saidrule' with letters rearranged gives 'RESIDUAL'.
(Other definitions for residual that I've seen before include "Leftover - used rail (anag)" , "Leftover; repeat royalty" , "Pertaining to what is left over" , "Remaining at the end" , "surplus" .)