Press cavalry to provide engine once (4,5)
I believe the answer is:
iron horse
'engine once' is the definition.
(poetic term for a steam engine)
'press cavalry' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'cavalry' becomes 'horse' (horse cavalry is a kind of cavalry).
'iron'+'horse'='IRON HORSE'
'to provide' is the link.
(Other definitions for iron horse that I've seen before include "film once in America!" , "one's gone off the rails" , "Old name for locomotive" , "stronger than the one left at Troy?" , "worn-out engine" .)