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 informally mean thank you).
'against' becomes 'v' (abbreviation for 'versus').
'online' becomes 'e' (as in e-mail or e-commerce).
'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" .)