Help out with game, oddly showing self-possession (6)
I believe the answer is:
phlegm
'self-possession' is the definition.
(thesaurus)
'help out with game oddly' is the wordplay.
'out with' is an anagram indicator (out can mean wrong or inaccurate).
'oddly' indicates alternate letters.
The alternating letters of 'game' are 'gm'.
'help' with its letters mixed up with 'gm' gives 'PHLEGM'.
'showing' acts as a link.
(Other definitions for phlegm that I've seen before include "Mucous secretion" , "Mucus - stolidity" , "Impassivity" , "One of the four humours" , "Composure" .)