Adult you and I hit is stunned (9)
I believe the answer is:
awestruck
'is stunned' is the definition.
(I've seen this before)
'adult you and i hit' is the wordplay.
'adult' becomes 'a'.
'you and i' becomes 'we'.
'hit' becomes 'struck' (I've seen this before**).
'a'+'we'+'struck'='AWESTRUCK'
(Other definitions for awestruck that I've seen before include "Showing a feeling of wonder and dread" , "reverential wonder" , "Shocked" , "Suck water (anag) - completely astonished" , "overcome by wonder" .)