Concert in timeless Tunis going ahead (6)
I believe the answer is:
unison
'concert' is the definition.
(to work in concert is to work in unison)
'timeless tunis going ahead' is the wordplay.
'timeless tunis going' becomes 'unis' (I am not sure about this - if you are sure you should believe this answer much more).
'ahead' becomes 'on' (synonyms).
'unis'+'on'='UNISON'
'in' is the link.
Can you help me to learn more?
(Other definitions for unison that I've seen before include "using same pitch" , "concert" , "As one voice" , "In agreement - like sound of choir" , "Public service trade union" .)