Like a cross country runner's home on Channel Islands housing police once (9)
I believe the answer is:
cruciform
'like a cross country runner's home' is the definition.
I can't tell whether this definition defines the answer.
'channel islands housing police once' is the wordplay.
'channel islands' becomes 'CI'.
'housing' means one lot of letters goes inside another.
'police' becomes 'ruc'.
'once' becomes 'form' (I can't explain this - if you can you should give a lot more credence to this answer).
'ci' enclosing 'ruc' is 'cruci'.
'cruci'+'form'='CRUCIFORM'
'on' is the link.
Can you help me to learn more?
(Other definitions for cruciform that I've seen before include "for use in cathedral maybe" , "Cross-shaped" , "Shaped like a cross" .)