Cold, say, for which one lines cape (7)
I believe the answer is:
illness
'cold say' is the definition.
(I know that cold is a type of communicable disease)
'one lines cape' is the wordplay.
'one' becomes 'i' (Roman numeral).
'lines' becomes 'll' (l is an abbreviation for line).
'cape' becomes 'ness' (synonyms).
'i'+'ll'+'ness'='ILLNESS'
'for which' acts as a link.
(Other definitions for illness that I've seen before include "Ailment, disorder" , "Malady, sickness" , "Disease, sickness" , "Period of sickness" , "Indisposition, sickness" .)