UNICEF is to change and it's catching (10)
I believe the answer is:
Here is my best explanation:
'it's catching' is the definition.
('infectious' can be similar in meaning to 'catching')
'unicef is to change' is the subsidiary indication.
'change' indicates anagramming the letters.
'unicefisto' with letters rearranged gives 'infectious'.
'and' is the link.
(Other definitions for infectious that I've seen before include "Catching", "Liable to spread disease", "easily picked up", "(Of disease) liable to spread", "Easily transmitted" and "(Of disease) catchable from others".)