Thunderous god descending on knight, old character (5)
I believe the answer is:
thorn
'old character' is the definition.
(letter used in Old English)
'thunderous god descending on knight' is the wordplay.
'thunderous god' becomes 'thor' (I've seen this before).
'descending on' says to put letters next to each other (in a down clue, some letters go on top of others).
'knight' becomes 'N' (chess abbreviation).
'thor'+'n'='THORN'
(Other definitions for thorn that I've seen before include "Source of irritation" , "North (anag) -- point" , "A vegetable spike" , "Sharp point; runic character" , "Sharp point on a shrub" .)