Turn of phrase in which one police inspector gets an order (5)
I believe the answer is:
idiom
'turn of phrase' is the definition.
(I've seen this before)
'inspector gets an order' is the wordplay.
'inspector' becomes 'di' (abbreviation for Detective Inspector).
'gets' indicates putting letters inside.
'an' becomes 'i' (Roman numeral for one).
'order' becomes 'om' (Order of Merit).
'i'+'om'='iom'
'di' going into 'iom' is 'IDIOM'.
'in which one police' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for idiom that I've seen before include "Phrase with a meaning that is not literal" , "Word usage peculiar to a specific group" , "Figure of speech" , "Peculiar term" , "Special linguistic usage" .)