Widespread infections gathered speed? I'm in charge, initially (9)
I believe the answer is:
epidemics
'widespread infections' is the definition.
'gathered speed? i'm in charge initially' is the wordplay.
'gathered' indicates an anagram.
'initially' indicates taking the first letters.
The first letters of 'in charge' is 'ic'.
'speed'+'im'+'ic'='speedimic'
'speedimic' is an anagram of 'EPIDEMICS'.
(Other definitions for epidemics that I've seen before include "Widespread outbreaks of an infectious diseases" , "outbreaks of disease" .)