Search for a long time (6)
I believe the answer is:
forage
'search' is the definition.
(I know that forage is a type of search)
'for a long time' is the wordplay.
'a long time' becomes 'age' (an age is a period of time).
'for'+'age'='FORAGE'
(Other definitions for forage that I've seen before include "Collect food" , "Go searching, perhaps for food" , "sort of cap" , "Rummage, search for" , "Search widely for food" .)