Cracking up in southeast river (5)
I believe the answer is:
super
'cracking' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'up in southeast river' is the wordplay.
'in' is an insertion indicator.
'southeast' becomes 'se' (synonyms).
'river' becomes 'r' (abbreviation).
'up' inserted inside 'se' is 'supe'.
'supe'+'r'='SUPER'
Can you help me to learn more?
(Other definitions for super that I've seen before include "Exceptionally good" , "Extremely good" , "Excellent; police officer" , "Delightful" , "(Of product) of extra fine quality" .)