Assemble at the railhead, mate (5)
I believe the answer is:
rally
'assemble' is the definition.
(I've seen this in another clue)
'at the railhead mate' is the wordplay.
'at' says to put letters next to each other.
'the railhead' becomes 'r' (head letter of 'rail').
'mate' becomes 'ally' (I've seen this before).
'r'+'ally' is 'RALLY'.
(Other definitions for rally that I've seen before include "Pick up" , "(Of shares) increase after a fall" , "Recover; sequence of shots" , "Mass meeting -- revive" , "Revive - motoring contest" .)