Non-stop aces badly restricting Seles (9)
I believe the answer is:
ceaseless
'non-stop' is the definition.
(I've seen this before)
'aces badly restricting seles' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'restricting' is an insertion indicator.
'aces' anagrammed gives 'ceas'.
'ceas' going around 'seles' is 'CEASELESS'.
(Other definitions for ceaseless that I've seen before include "keeps on running" , "Incessant, unremitting" , "Continuous" , "Interminable" , "Non-stop" .)