Illicit burden athlete dealt with (5,3,5)
I believe the answer is:
under the table
'illicit' is the definition.
(thesaurus)
'burden athlete dealt with' is the wordplay.
'dealt with' indicates anagramming the letters.
'burden'+'athlete'='burdenathlete'
'burdenathlete' is an anagram of 'UNDER-THE-TABLE'.
(Other definitions for under the table that I've seen before include "secretly" , "Transacted in secret - sozzled" , "footsie may be played here" , "at the bottom of the list" , "Dodgy" .)