Seek on behalf of, taking a long time (6)
I believe the answer is:
forage
'seek' is the definition.
(I've seen this before)
'on behalf of taking a long time' is the wordplay.
'on behalf of' becomes 'for' (eg speaking on behalf of or for someone else).
'taking' says to put letters next to each other.
'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 "Plunder; rummage" , "Rummage, go searching" , "Go searching, perhaps for food" , "look around" , "Look for food" .)