Pole having illness that's catching (7)
I believe the answer is:
nailing
'catching' is the definition.
(I've seen this before)
'pole having illness' is the wordplay.
'pole' becomes 'n' (abbreviation for North, as in North Pole).
'having illness' becomes 'ailing' (I am not sure about this - if you are sure you should believe this answer much more).
'n'+'ailing'='NAILING'
'that's' is the link.
Can you help me to learn more?
(Other definitions for nailing that I've seen before include "Securely pinning" , "Securing" , "Tacking" , "Catching" , "Fixing" .)