Boy, caught by stream, a source of irritation (7)
I believe the answer is:
Here is my best explanation:
'a source of irritation' is the definition.
I can't tell whether this defines the answer.
'boy caught by stream' is the wordplay.
'boy' becomes 'son' (synonyms).
'caught' indicates a 'sounds like' (homophone) clue (to catch something can mean to hear it).
'by stream' becomes 'burn' (a burn is a small stream. I am not sure about the 'by' bit.).
'son' sounds like 'sun'.