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 can't justify this - if you can you should believe this answer much more).
'n'+'ailing'='NAILING'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for nailing that I've seen before include "Securely pinning" , "Securing" , "Tacking" , "Catching" , "Fixing" .)