Letting up fast during terrible reign (9)
I believe the answer is:
relenting
'letting up' is the definition.
(I've seen this before)
'fast during terrible reign' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'during' means one lot of letters goes inside another.
'terrible' is an anagram indicator.
'reign' with letters rearranged gives 'reing'.
'lent' going within 'reing' is 'RELENTING'.
(Other definitions for relenting that I've seen before include "Becoming less severe" , "Giving way" , "Letting up" , "Growing less severe" , "Showing less severity" .)