Capaciousness of old pit traversed by Scottish explorer (9)
I believe the answer is:
roominess
'capaciousness' is the definition.
(synonyms)
'old pit traversed by scottish explorer' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pit' becomes 'mine' (mining pit).
'traversed by' indicates putting letters inside.
'scottish explorer' becomes 'Ross' (19th-century explorer John Ross).
'o'+'mine'='omine'
'omine' inserted into 'ross' is 'ROOMINESS'.
'of' is the link.
(Other definitions for roominess that I've seen before include "near-emptyness of space" , "Capaciousness" .)