One apprehending eclipse over rocky hill (6)
I believe the answer is:
captor
'one apprehending' is the definition.
(someone who catches or apprehends)
'eclipse over rocky hill' is the wordplay.
'eclipse' becomes 'cap' (cap can mean to eclipse or outdo).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'rocky hill' becomes 'tor' (I've seen this before).
'cap'+'tor'='CAPTOR'
(Other definitions for captor that I've seen before include "One takes prisoners" , "One who confines another" , "this one may have led some into a trap" , "Holder of a prisoner" , "To carp (anag)" .)