Neighbour inhaling oxygen roughly (5)
I believe the answer is:
about
'roughly' is the definition.
(about can mean roughly or approximately)
'neighbour inhaling oxygen' is the wordplay.
'neighbour' becomes 'abut' (I've seen this before).
'inhaling' is an insertion indicator.
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'abut' placed around 'o' is 'ABOUT'.
(Other definitions for about that I've seen before include "'All - - Eve' (film)" , "In the area of or in relation to" , "nearby" , "in the offing?" , "Circa" .)