Bar on train where travellers might be held up (10)
I believe the answer is:
checkpoint
'where travellers might be held up' is the definition.
(place where travellers may be stopped)
'bar on train' is the wordplay.
'bar' becomes 'check' (check can mean to bar or control).
'on' means one lot of letters go next to another.
'train' becomes 'point' (both can mean to aim).
'check'+'point'='CHECKPOINT'
(Other definitions for checkpoint that I've seen before include "Where on inspection one may cross" , "border crossing?" , "where travellers are inspected" , "place at frontier?" , "Charlie, perhaps" .)