Liqueur excessive queen and a mother knocked back (8)
I believe the answer is:
amaretto
'liqueur' is the definition.
(I've seen this before)
'excessive queen and a mother knocked back' is the wordplay.
'excessive' becomes 'ott' (abbreviation for 'over the top').
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'and' says to put letters next to each other.
'mother' becomes 'ma' (ma can mean mother).
'knocked back' shows that the letters should be reversed in order.
'ott'+'er'+'a'+'ma'='otterama'
'otterama' back-to-front is 'AMARETTO'.
(Other definitions for amaretto that I've seen before include "Almond-flavoured liqueur" , "Italian almond liqueur" , "Ma, rotate the almond liqueur" , "drink" , "rink" .)