They're against an unexpected initial success (4)
I believe the answer is:
odds
'they're' is the definition.
The definition suggests an adverb but the answer is not.
'against an unexpected initial success' is the wordplay.
'against' says to put letters next to each other.
'an unexpected' becomes 'odd' (odd can mean unexpected or unusual).
'initial' indicates taking the first letters.
The initial letter of 'success' is 's'.
'odd' next to 's' is 'ODDS'.
Can you help me to learn more?
(Other definitions for odds that I've seen before include "Bookies give them" , "Scraps" , "Chances of occurrence" , "Perhaps even money" , "prices" .)