Each one was first among equals, really (5)
I believe the answer is:
every
'each one' is the definition.
(I've seen this before)
'equals really' is the wordplay.
'equals' becomes 'e' (this might be a standard abbreviation which I don't know about).
'really' becomes 'very' (I've seen this before).
'e'+'very'='EVERY'
'was first among' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for every that I've seen before include "All possible; the utmost" , "Each (one of a set)" , "Each of a number" , "Each single (item)" , "As much as possible" .)