Quite unbelievable play's screened (7)
I believe the answer is:
totally
'quite' is the definition.
(I've seen this before)
'unbelievable play's screened' is the wordplay.
I cannot really see how this works, but
'unbelievable' could be 'tall' (e.g. 'a tall story') and 'tall' is located in the answer.
'play' could be 'toy' (synonyms) and 'toy' is found in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for totally that I've seen before include "Clean" , "Wholly" , "To the full extent" , "Utterly" , "without reservation" .)