Power for first of trains in line (5)
I believe the answer is:
steam
'power' is the definition.
(thesaurus)
'first of trains in line' is the wordplay.
'first of' suggests taking the first letters.
'in' indicates putting letters inside.
'line' becomes 'seam' ('seam' can be a synonym of 'line'**).
The initial letter of 'trains' is 't'.
't' inserted into 'seam' is 'STEAM'.
'for' is the link.
(Other definitions for steam that I've seen before include "Driving force" , "comes with a hiss?" , "Sort of water" , "It's water mist, mates" , "get very angry" .)