Appear to secure - say - third of surplus space (7)
I believe the answer is:
legroom
'space' is the definition.
(I know that legroom is a type of space)
'appear to secure say third of surplus' is the wordplay.
'appear' becomes 'loom' (looming is a kind of appearing).
'to secure' is an insertion indicator (inserted letters are secured inside).
'say' becomes 'eg' (both can mean 'for example').
'third of surplus' becomes 'r' (ord letter of 'surplus').
'eg'+'r'='egr'
'loom' enclosing 'egr' is 'LEGROOM'.
(Other definitions for legroom that I've seen before include "(Seated) person's space for feet" , "Space in front of a seat" , "Space to stretch for seated person" , "Space often lacking on a plane!" , "Space needed to avoid cramped position" .)