Be too lavish because of a party? (6)
I believe the answer is:
overdo
'be too lavish' is the definition.
I can't tell whether this definition defines the answer.
'because of a party?' is the wordplay.
'because of' becomes 'over' (eg 'he's upset over it').
'a party?' becomes 'do' (do can informally mean a party).
'over'+'do'='OVERDO'
Can you help me to learn more?
(Other definitions for overdo that I've seen before include "Be excessive, say cook too long" , "Carry to excess" , "Exaggerate; use too much of" , "Carry too far" , "Use to excess" .)