Last word in steam engines (4)
I believe the answer is:
amen
'last word' is the definition.
(I've seen this before)
'in steam engines' is the wordplay.
'in' indicates a hidden word.
'AMEN' can be found hidden inside 'steam engines'.
(Other definitions for amen that I've seen before include "Word of agreement" , "statement of affirmation" , "Extremist" , "Word of approval" , "prayer's ending" .)