Current number (over 50) conforming with the rules (2,4)
I believe the answer is:
in line
'conforming with the rules' is the definition.
I can't tell whether this definition defines the answer.
'current number over 50' is the wordplay.
'current' becomes 'I' (symbol for electric current).
'number' becomes 'nine' (nine is a number).
'over' is an insertion indicator.
'50' becomes 'l' (Roman numeral).
'nine' going around 'l' is 'nline'.
'i'+'nline'='IN-LINE'
Can you help me to learn more?
(Another definition for in line that I've seen is " Under control; aligned".)