Any number with illness that's catching (7)
I believe the answer is:
nailing
'catching' is the definition.
(I've seen this before)
'any number with illness' is the wordplay.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'with' is a charade indicator (letters next to each other).
'illness' becomes 'ailing' (I can't explain this - if you can you should give a lot more credence to this answer).
'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 "Catching" , "Securely pinning" , "Fixing" , "Tacking" , "Securing" .)