Hunt for a long time (6)
I believe the answer is:
Here is my best explanation:
'hunt' is the definition.
(forage is a kind of hunt)
'for a long time' is the subsidiary indication.
'a long time' becomes 'age' ('age' can be a synonym of 'long time'**).
(Other definitions for forage that I've seen before include "Search around for food", "Collect food", "Hunt for food or provisions", "Search widely for food", "Search, rummage", "Go searching, rummage", "Wander and feed", "Plunder; rummage", "Rummage, search for" and "Seek for food".)