Train’s audible signal (5)
I believe the answer is:
queue
'train's' is the definition.
(thesaurus)
'audible signal' is the wordplay.
'audible' indicates a 'sounds like' (homophone) clue.
'signal' becomes 'cue' (I've seen this before).
'cue' sounds like 'QUEUE'.
(Other definitions for queue that I've seen before include "orderly people?" , "Waiting list" , "Line of people; pigtail" , "Buyers' line" , "Pigtail; get in line" .)