Reduce European investment in party before team become old-fashioned (9)

'reduce' is the definition.
(to reduce the proportion of oxygen in chemistry)

'european investment in party before team become old-fashioned' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'investment in' indicates putting letters inside.
'party before' becomes 'do' (do is a kind of party. I am not sure about the 'before' bit.).
'team' becomes 'xi' (eleven, a football team).
'become old-fashioned' becomes 'date' (to date is to become old-fashioned).
'e' placed into 'do' is 'deo'.
'deo'+'xi'+'date'='DEOXIDATE'