Assemble at the railhead, mate (5)
I believe the answer is:
rally
'assemble' is the definition.
(thesaurus)
'at the railhead mate' is the wordplay.
'at' is a charade indicator (letters next to each other).
'the railhead' becomes 'r' (head letter of 'rail').
'mate' becomes 'ally' (I've seen this before).
'r' put next to '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" .)