Detective about to have good look, reportedly, closer to cellar? (10)
I believe the answer is:
downstairs
'closer to cellar?' is the definition.
(closer to the ground or a cellar)
'detective about to have good look' is the wordplay.
'detective' becomes 'DS' (detective sergeant).
'about' means one lot of letters goes inside another.
'to have' becomes 'own' (synonyms).
'good look' becomes 'stair' (I can't justify this - if you can you should give a lot more credence to this answer).
'own'+'stair'='ownstair'
'ds' going around 'ownstair' is 'DOWNSTAIRS'.
'reportedly' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
(Other definitions for downstairs that I've seen before include "in the cellar?" , "towards lower levels" , "On a lower floor" , "On ground floor (or in basement)" .)