Sickly Martha's lacking resistance to condition (6)
I believe the answer is:
asthma
'condition' is the definition.
(asthma is a kind of condition)
'sickly martha's lacking resistance' is the wordplay.
'sickly' is an anagram indicator (I've seen this in other clues).
'lacking' suggests deleting specific letters.
'resistance' becomes 'r' (electronics abbreviation).
'marthas' with 'r' taken out is 'mathas'.
'mathas' is an anagram of 'ASTHMA'.
'to' acts as a link.
(Other definitions for asthma that I've seen before include "Strange that Mat has difficulty breathing" , "Breathing illness" , "Respiratory condition" , "Chronic breathing condition" , "Respiratory disorder with wheezing" .)