Many times, Don felt terrible (7)
I believe the answer is:
tenfold
'many times' is the definition.
'tenfold' can be an answer for 'times' (I have seen 'several times' mean 'tenfold' so perhaps 'times' could also mean 'tenfold'). I'm not sure about the 'many' bit.
'don felt terrible' is the wordplay.
'terrible' indicates an anagram.
'don'+'felt'='donfelt'
'donfelt' with letters rearranged gives 'TENFOLD'.
Can you help me to learn more?
(Other definitions for tenfold that I've seen before include "several times" , "(Increased) by an order of magnitude" , "An order of magnitude as much" , "x10" , "(Multiplied) by putting 0 on end" .)