Order 51 signs (5)
I believe the answer is:
obeli
'signs' is the definition.
(I've seen this before)
'order 51' is the wordplay.
'order' becomes 'OBE' (Order of the British Empire award).
'51' becomes 'LI' (Roman numerals).
'obe'+'li'='OBELI'
(Other definitions for obeli that I've seen before include "Dagger signs for footnotes" , "signs of suspect text" , "Little daggers" , "Dagger signs in printing" , "Signs of death" .)