Roman emperor, hiding in enclosure, secured position (8)
I believe the answer is:
foothold
'secured position' is the definition.
(secure position for the foot when climbing)
'roman emperor hiding in enclosure' is the wordplay.
'roman emperor' becomes 'Otho' (Otho was briefly Roman emperor in 69 AD).
'hiding in' means one lot of letters goes inside another.
'enclosure' becomes 'fold' (fold is a kind of enclosure).
'otho' inserted within 'fold' is 'FOOTHOLD'.
(Other definitions for foothold that I've seen before include "Small area captured and held as the basis for subsequent advance" , "Initial position from which to make progress" , "Secure first position" , "Secure position from which progress can be made" , "Purchase" .)