If upset and cross, all you need is some dope! (3)
I believe the answer is:
fix
'all you need is some dope' is the definition.
I can't judge whether this defines the answer.
'if upset and cross' is the wordplay.
'upset' says the letters should be written in reverse.
'and' is a charade indicator (letters next to each other).
'cross' becomes 'x' (X is cross-shaped).
'if' back-to-front is 'fi'.
'fi'+'x'='FIX'
Can you help me to learn more?
(Other definitions for fix that I've seen before include "Addict's dose of drug" , "mess" , "Repair or attach" , "Mend" , "Fasten" .)