Popular iron tonic treated contagion (9)
I believe the answer is:
infection
'contagion' is the definition.
(I know that contagion can be written as infection)
'popular iron tonic treated' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'iron' becomes 'Fe' (chemical symbol for iron).
'treated' indicates an anagram.
'tonic' is an anagram of 'ction'.
'in'+'fe'+'ction'='INFECTION'
(Other definitions for infection that I've seen before include "what may be caught" , "Process of catching virus" , "Invasion of the body by pathogenic micro-organisms" , "Invasion by pathogenic micro-organism" , "Disease caught" .)