Bad sport inhibiting one's cheers (6)
I believe the answer is:
prosit
'cheers' is the definition.
(I've seen this before)
'bad sport inhibiting one's' is the wordplay.
'bad' is an anagram indicator.
'inhibiting' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'sport' with letters rearranged gives 'prost'.
'prost' enclosing 'i' is 'PROSIT'.
(Other definitions for prosit that I've seen before include "I sport during German toast to your health" , "Good luck to you" , "(German for) cheers!" , "All the best" , "I sport with a drinking toast in Germany" .)