Current, with temperature dropping a little, is a type of illusion (7)
I believe the answer is:
optical
'a type of illusion' is the definition.
(optical illusion is a type of illusion)
'current with temperature dropping a little' is the wordplay.
I cannot really understand how this works, but
'current' could be 'i' (symbol for electric current) and 'i' is found in the answer.
'temperature' could be 't' (abbreviation) and 't' is present in the answer.
'a' is within the answer.
This explanation may well be incorrect...
'is' is the link.
(Other definitions for optical that I've seen before include "Coal tip (anag)" , "Involving light" , "Working with light" , "Relating to vision and light" , "Concerning light" .)