March past's rear broke from the right (5)
I believe the answer is:
troop
'march' is the definition.
(I know that troop is a kind of march)
'past's rear broke from the right' is the wordplay.
'rear' says to take the final letters (the back of the word).
'broke' becomes 'poor' (similar in meaning).
'from the right' says the letters should be written in reverse.
The last letter of 'past' is 't'.
'poor' back-to-front is 'roop'.
't'+'roop'='TROOP'
(Other definitions for troop that I've seen before include "(Of people) come or go in large numbers" , "Military group" , "Band of people" , "Flock; Scout group" , "Flock, crowd" .)