A prop? Not at sea (6)
I believe the answer is:
ashore
'not at sea' is the definition.
(I've seen this in another clue)
'a prop?' is the wordplay.
'prop?' becomes 'shore' (shore can mean to prop up or support).
'a'+'shore'='ASHORE'
(Other definitions for ashore that I've seen before include "in port?" , "Going on land from sea" , "On land (as opposed to the sea)" , "Toward land from water" , "Hoarse being no longer at sea" .)