Foster finally made Byzantine castle in Spain (5)
I believe the answer is:
dream
'castle in spain' is the definition.
(I've seen this in another clue)
'foster finally made byzantine' is the wordplay.
'finally' says to take the final letters.
'byzantine' indicates anagramming the letters (byzantine means complex or confusing).
The final letter of 'foster' is 'r'.
'r'+'made'='rmade'
'rmade' with letters rearranged gives 'DREAM'.
(Other definitions for dream that I've seen before include "Something imaginary" , "Night-time fantasy" , "Sleeping thoughts" , "Aspire" , "Armed (anag)" .)