One feeling bad at poorly patient (13)
I believe the answer is:
indefatigable
'patient' is the definition.
(thesaurus)
'one feeling bad at poorly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'poorly' is an anagram indicator.
'feeling'+'bad'+'at'='feelingbadat'
'feelingbadat' is an anagram of 'ndefatigable'.
'i'+'ndefatigable'='INDEFATIGABLE'
(Other definitions for indefatigable that I've seen before include "Unremitting" , "Assiduous" , "Tireless" , "Persistent" , "Unflagging" .)