Cider cocktail includes the last of the scrumpy - it's provides a special effect (3,3)
I believe the answer is:
dry ice
'effect' is the definition.
(I have seen 'theatrical effect' mean 'dry ice' so perhaps 'effect' could also mean 'dry ice')
'cider cocktail includes the last of the scrumpy' is the wordplay.
'cocktail' indicates an anagram.
'includes' means one lot of letters goes inside another.
'the last of' indicates one should take the final letters.
The last letter of 'scrumpy' is 'y'.
'cider' anagrammed gives 'drice'.
'drice' enclosing 'y' is 'DRY ICE'.
'it's provides a special' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dry ice that I've seen before include "White mist on stage" , "Carbon dioxide snow" , "Solidified CO2" , "Carbon dioxide vapour used on stage" , "theatrical effect" .)