Shaker town holding back migration (4)
I believe the answer is:
trek
'migration' is the definition.
(both can mean a long journey)
'shaker town holding back' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'back' says the letters should be written backwards.
'kert' can be found hidden inside 'shaker town'.
'kert' backwards is 'TREK'.
(Other definitions for trek that I've seen before include "Long and difficult trip" , "Expedition or difficult trip" , "A long hard march" , "Long difficult walk" , "Slog" .)