A particular turn of phrase: "I had" - on the Isle of Man (5)
I believe the answer is:
idiom
'a particular turn of phrase' is the definition.
I can't judge whether this defines the answer.
'i had on the isle of man' is the wordplay.
'i had' becomes 'i'd' (I'd).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'the isle of man' becomes 'iom'.
'id'+'iom'='IDIOM'
Can you help me to learn more?
(Other definitions for idiom that I've seen before include "Peculiarity of phrase in a language" , "Phrase such as over the moon" , "Linguistic peculiarity" , "Colloquialism" , "Expression that does not make literal sense" .)