Not wise to collar old woman regarding choice of expression (9)
I believe the answer is:
idiomatic
'expression' is the definition.
(idiomatic expression is a kind of expression)
'not wise to collar old woman regarding choice' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'woman' could be 'di' (short for Diana) and 'di' is found in the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for idiomatic that I've seen before include "spoken like a native" , "Colloquial" , "vernacular" , "Using expressions natural to a native speaker" , "Expressed in distinctive or metaphorical way" .)