Practicable involving annual return capable of change (8)
I believe the answer is:
variable
'capable of change' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'practicable involving annual return' is the wordplay.
'practicable' becomes 'viable' (synonyms).
'involving' indicates putting letters inside.
'annual return' becomes 'ar' (this could be a standard abbreviation which I don't know about).
'viable' going around 'ar' is 'VARIABLE'.
Can you help me to learn more?
(Other definitions for variable that I've seen before include "can be changed" , "Not steady or regular" , "mathematician's expression" , "Unsteady" , "Not consistent, without a fixed pattern" .)