Dealer's opening boat business — we're told it could occupy a lot of space (4,6)
I believe the answer is:
dark matter
'we're told it could occupy a lot of space' is the definition.
(matter thought to exist in space)
'dealer's opening boat business' is the wordplay.
'opening' suggests taking the first letters.
'boat' becomes 'ark' (ark is a kind of boat).
'business' becomes 'matter' (business can mean some matter or situation).
The initial letter of 'dealers' is 'd'.
'd'+'ark'+'matter'='DARK MATTER'
(Other definitions for dark matter that I've seen before include "A large proportion of the mass of the universe" , "In astronomy, theoretical non-luminous material" .)