In the pub, a tonic for staff (5)
I believe the answer is:
baton
'staff' is the definition.
(thesaurus)
'in the pub a tonic' is the wordplay.
'in' indicates a hidden word.
'BATON' can be found hidden inside 'pub a tonic'.
'for' acts as a link.
(Other definitions for baton that I've seen before include "Police (or conductor's) stick" , "Relay race stick" , "one watched by players" , "Relay runner's stick" , "A conductor wields it - or a policemen" .)