Assert old medical condition limits resistance (5)
I believe the answer is:
argue
'assert' is the definition.
(thesaurus)
'old medical condition limits resistance' is the wordplay.
I cannot really understand how this works, but
'resistance' could be 'r' (abbreviation in physics and electronics) and 'r' is located in the answer.
'condition' could be 'ague' (ague is a kind of condition) and 'ague' is found within the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for argue that I've seen before include "Discuss, contend" , "Dispute, express angry words" , "Dispute - debate" , "Contest verbally, dispute" , "express disagreement" .)