Dad’s Army cast entertaining English illusions (9)
I believe the answer is:
daydreams
'illusions' is the definition.
'dad's army cast entertaining english' is the wordplay.
'cast' is an anagram indicator (letters are dispersed or cast around).
'entertaining' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'dads'+'army'='dadsarmy'
'dadsarmy' with letters rearranged gives 'daydrams'.
'daydrams' placed around 'e' is 'DAYDREAMS'.
(Other definitions for daydreams that I've seen before include "Reveries" , "Idly imagines" , "hopeful aspirations?" , "Fantasise while awake" .)