Engine assembled for son or heir (4,5)
I believe the answer is:
iron horse
'engine' is the definition.
(iron horse is a kind of engine)
'assembled for son or heir' is the wordplay.
'assembled for' indicates anagramming the letters.
'son'+'or'+'heir'='sonorheir'
'sonorheir' with letters rearranged gives 'IRON HORSE'.
(Other definitions for iron horse that I've seen before include "Train leader" , "worn-out engine" , "People made tracks for this!" , "film once in America!" , "stronger than the one left at Troy?" .)