Reason partners enter cathedral area (5)
I believe the answer is:
sense
'reason' is the definition.
(I've seen this in another clue)
'partners enter cathedral area' is the wordplay.
'partners' becomes 'ns' (North and South are partners in the game bridge).
'enter' is an insertion indicator.
'cathedral area' becomes 'see' (see is a kind of area. I am not sure about the 'cathedral' bit.).
'ns' placed inside 'see' is 'SENSE'.
Can you help me to learn more?
(Other definitions for sense that I've seen before include "Smell, say" , "Divine" , "Get an impression" , "Detect, be aware of" , "Could be common or horse" .)