Expert in tongue ailment joining ENT (6)
I believe the answer is:
fluent
'expert in tongue' is the definition.
(I've seen this before)
'ailment joining ent' is the wordplay.
'ailment' becomes 'flu' (I've seen this in another clue).
'joining' is a charade indicator (letters next to each other).
'flu'+'ent'='FLUENT'
(Other definitions for fluent that I've seen before include "Able to speak smoothly and readily" , "Free-flowing" , "Smooth and unconstrained in speech or movement" , "Competent (of language) - graceful (of movement)" , "Of speech, smooth and eloquent" .)