Pub in Cheers against online service (6)
I believe the answer is:
tavern
'pub' is the definition.
(a tavern is a pub or inn)
'cheers against online service' is the wordplay.
'cheers' becomes 'ta' (both can mean thank you).
'against' becomes 'v' (abbreviation for 'versus').
'online' becomes 'e' (abbreviation for electronic, e.g. in ecommerce).
'service' becomes 'RN' (abbreviation for Royal Navy).
'ta'+'v'+'e'+'rn'='TAVERN'
'in' acts as a link.
(Other definitions for tavern that I've seen before include "Old song: 'There is a ...... in the town, in the town'" , "An old English pub" , "Tap house" , "Local" , "Tap-house" .)