...eagerness, clarity affected, falling behind midway (8)
I believe the answer is:
alacrity
'falling behind midway' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'eagerness clarity affected' is the wordplay.
'eagerness' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'affected' is an anagram indicator.
'clarity' with letters rearranged gives 'lacrity'.
'a'+'lacrity'='ALACRITY'
Can you help me to learn more?
(Other definitions for alacrity that I've seen before include "Prompt enthusiasm" , "Quickness" , "Brisk and cheerful readiness" , "prompt response" , "'Quick, cheerful enthusiasm (8)'" .)