In spite of everything, they're oddly entering a decline (5,3)
I believe the answer is:
after all
'in spite of everything' is the definition.
(I've seen this in another clue)
'they're oddly entering a decline' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'entering' means one lot of letters goes inside another.
'decline' becomes 'fall' (synonyms).
The alternating letters of 'theyre' are 'ter'.
'a'+'fall'='afall'
'ter' inserted inside 'afall' is 'AFTER ALL'.
(Other definitions for after all that I've seen before include "'In spite of what's been said and done (5,3)'" , "in the final reckoning?" , "Nevertheless, when everything has been considered" , "in spite of expectations to the contrary" , "at the end of the day" .)