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 "Hunt for food or provisions", "Collect food", "Rummage, go searching", "Search around for food", "Plunder; rummage", "Go searching, rummage", "Search widely for food", "Look for food", "Wander and feed" and "Search, rummage".)