A foolish sort had to make trouble (7)
I believe the answer is:
agitate
'make trouble' is the definition.
(I've seen this in another clue)
'a foolish sort had' is the wordplay.
'foolish sort' becomes 'git' (I can't explain this - if you can you should believe this answer much more).
'had' becomes 'ate' (e.g. I had an ice cream).
'a'+'git'+'ate'='AGITATE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for agitate that I've seen before include "Make nervous; stir" , "Keep moving" , "Disturb or churn" , "Unsettle" , "Stir; arouse" .)