Straighten out Charlie, humorist at university (5,2)
I believe the answer is:
clear up
'straighten out' is the definition.
('clear up' can be a synonym of 'straighten out')
'charlie humorist at university' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'humorist' becomes 'lear' (Edward Lear).
'at university' becomes 'up'.
'c'+'lear'+'up'='CLEAR-UP'
(Other definitions for clear up that I've seen before include "Solve a mystery - become brighter" , "(Of bad weather) improve" , "Tidy; explain; cure" , "be less untidy" , "Sort out" .)