Eagle needs to cover valley (4)
I believe the answer is:
glen
'valley' is the definition.
(I know that glen is a type of valley)
'eagle needs to cover' is the wordplay.
'to cover' says the answer is hidden in the clue (I've seen 'covered' mean this (the answer is covered up by the clue)).
'GLEN' is hidden in the letters of 'eagle needs'.
(Other definitions for glen that I've seen before include "Depression in Scotland" , "Scottish mountain valley" , "Narrow Scots valley" , "Scottish or Irish valley" , "Narrow, deep mountain valley" .)