Crotchety head teacher finds source had good intentions (5,4)
I believe the answer is:
meant well
'had good intentions' is the definition.
(I've seen this in another clue)
'crotchety head teacher finds source' is the wordplay.
'crotchety head teacher finds' becomes 'meant' (I can't justify this - if you can you should believe this answer much more).
'source' becomes 'well' (well is a kind of source).
'meant'+'well'='MEANT WELL'
Can you help me to learn more?
(Another definition for meant well that I've seen is " Intended to help".)