One laying track for a green engine (5)
I believe the answer is:
henry
'a green engine' is the definition.
(Thomas the Tank Engine character)
'one laying track' is the wordplay.
'one laying' becomes 'hen'.
'track' becomes 'ry'.
'hen'+'ry'='HENRY'
'for' is the link.
(Other definitions for henry that I've seen before include "- James, US-born novelist (The Turn of the Screw)" , "Author of The Turn of the Screw" , "Unit of inductance; man's name" , "CARRY ON FILM" , "O. -; US writer" .)