Knocked back very sweet old sherry (7)
I believe the answer is:
oloroso
'sherry' is the definition.
(I've seen this before)
'knocked back very sweet old' is the wordplay.
'knocked back' says the letters should be written in reverse.
'very' becomes 'so' (both can mean 'extremely').
'sweet' becomes 'rolo' (I am not sure about this - if you are sure you should believe this answer much more).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'so'+'rolo'='sorolo'
'sorolo' in reverse letter order is 'oloros'.
'oloros'+'o'='OLOROSO'
Can you help me to learn more?
(Other definitions for oloroso that I've seen before include "Dark, sweet sherry" , "Heavy dark sherry" , "Albert -, missionary, musician, philosopher" , "Drink" , "Medium-sweet sherry" .)