Hit hard by decaying portal (9)
I believe the answer is:
threshold
'portal' is the definition.
(thesaurus)
'hit hard by decaying' is the wordplay.
'hit hard' becomes 'thresh'.
'by' says to put letters next to each other.
'decaying' becomes 'old'.
'thresh'+'old'='THRESHOLD'
(Other definitions for threshold that I've seen before include "Doorstep" , "Lower limit" , "Beginning" , "gate" , "Verge" .)