Lack of success oddly feasible (4)
I believe the answer is:
fail
'lack of success' is the definition.
'fail' can be an answer for 'lack' (thesaurus). I am not sure about the 'of success' bit.
'oddly feasible' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternating letters of 'feasible' are 'FAIL'.
Can you help me to learn more?
(Other definitions for fail that I've seen before include "Neglect (to do something)" , "collapse" , "Go bankrupt" , "Stop working properly" , "Run out; flop" .)