Locomotive power line around train's front (5)
I believe the answer is:
steam
'locomotive power' is the definition.
(I've seen this in another clue)
'line around train's front' is the wordplay.
'line' becomes 'seam' (synonyms).
'around' means one lot of letters goes inside another.
'front' suggests taking the first letters.
The first letter of 'trains' is 't'.
'seam' going around 't' is 'STEAM'.
(Other definitions for steam that I've seen before include "enthusiasm" , "Pre-digital" , "What powered old engines" , "Source of power" , "Type of power" .)