The last word in steam engines (4)
I believe the answer is:
amen
'the last word' is the definition.
(I've seen this before)
'in steam engines' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AMEN' can be found hidden inside 'steam engines'.
(Other definitions for amen that I've seen before include "Response to prayer" , "Prayerful last word" , "statement of affirmation" , "Name for prayer's end" , "Name for end to prayer" .)