Fail to return a lot of anger, lacking courage (8)
I believe the answer is:
timorous
'lacking courage' is the definition.
(I've seen this before)
'fail to return a lot of anger' is the wordplay.
'fail' becomes 'omit' (I've seen this before).
'to return' says the letters should be written in reverse.
'a lot of anger' becomes 'rous' (I can't justify this - if you can you should give a lot more credence to this answer).
'omit' reversed gives 'timo'.
'timo'+'rous'='TIMOROUS'
Can you help me to learn more?
(Other definitions for timorous that I've seen before include "Not bold, nervously hesitant" , "Shy; retiring" , "Nervous, fearful" , "showing fear" , "Faint-hearted" .)