Kept up refusal in accepting L is fifth letter (7)
I believe the answer is:
epsilon
'fifth letter' is the definition.
(I've seen this before)
'kept up refusal in accepting l is' is the wordplay.
'kept up' shows that the letters should be reversed in order (in a down clue, letters go up).
'refusal in' becomes 'nope' (I can't justify this - if you can you should give a lot more credence to this answer).
'accepting' indicates putting letters inside (inserted letters are accepted).
'l'+'is'='lis'
'nope' going around 'lis' is 'nolispe'.
'nolispe' back-to-front is 'EPSILON'.
Can you help me to learn more?
(Other definitions for epsilon that I've seen before include "foreign letter" , "Foreign character" , "Fifth letter of the Greek alphabet" , "Lose pin (anag.)" , "Fifth brightest star in a constellation" .)