50 in 500 sheets identifying sphere of action (5)
I believe the answer is:
realm
'sphere of action' is the definition.
'realm' can be an answer for 'sphere' (synonyms). I'm not certain of the 'of action' bit.
'50 in 500 sheets' is the wordplay.
'50' becomes 'l' (Roman numeral).
'in' means one lot of letters goes inside another.
'500 sheets' becomes 'ream' (I've seen this in another clue).
'l' put within 'ream' is 'REALM'.
'identifying' acts as a link.
Can you help me to learn more?
(Other definitions for realm that I've seen before include "monarch's responsibility" , "Province" , "Area of interest/activity" , "Empire - state" , "Dominion" .)