Once ref roughed up by Royal Engineers, impose rule again (9)
I believe the answer is:
reenforce
'impose rule again' is the definition.
I can't judge whether this definition defines the answer.
'once ref roughed up by royal engineers' is the wordplay.
'roughed up' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'royal engineers' becomes 'RE' (abbreviation).
'once'+'ref'='onceref'
'onceref' with letters rearranged gives 'enforce'.
'enforce' after 're' is 'REENFORCE'.
Can you help me to learn more?
(Other definitions for reenforce that I've seen before include "Again compel compliance with (law, rule)" , "Impose again" .)