Excellent marijuana on reflection, starting to feel cheerful (3,6)
I believe the answer is:
top flight
'excellent' is the definition.
(I've seen this before)
'marijuana on reflection starting to feel cheerful' is the wordplay.
'marijuana' becomes 'pot' (slang term for marijuana).
'on reflection' shows that the letters should be reversed in order.
'starting to' indicates taking the first letters.
'cheerful' becomes 'light'.
The first letter of 'feel' is 'f'.
'pot' reversed gives 'top'.
'top'+'f'+'light'='TOP-FLIGHT'
(Other definitions for top flight that I've seen before include "This could afford access to penthouse" , "Of the highest class" , "outstanding" , "so good?" , "First-class" .)