Uninterrupted viral infection advanced with leader nowhere to be seen (6)
I believe the answer is:
fluent
'uninterrupted' is the definition.
(as in speaking a language without hesitations)
'viral infection advanced with leader nowhere to be seen' is the wordplay.
I cannot quite understand how this works, but
'infection' could be 'flu' (I've seen this before) and 'flu' is present in the answer.
This explanation may well be incorrect...
(Other definitions for fluent that I've seen before include "Flowing freely" , "Eloquent, silver-tongued" , "Smooth and unconstrained in speech or movement" , "Fully competent in a language" , "Speaking with facility" .)