Hopeful prohibition gets lifted during decline (7)
I believe the answer is:
wannabe
'hopeful' is the definition.
(I know that hopeful can be written as wannabe)
'prohibition gets lifted during decline' is the wordplay.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'gets lifted' shows that the letters should be reversed in order (in a down clue, letters go up).
'during' indicates putting letters inside.
'decline' becomes 'wane' (wane is a kind of decline).
'ban' backwards is 'nab'.
'nab' inserted into 'wane' is 'WANNABE'.
(Other definitions for wannabe that I've seen before include "Aspiring film star possibly" , "hopeful" , "One with hopes" , "Ambitious person" , "One aiming (ineffectively) at a certain style etc" .)