Old cronies abandoned leader and fell into disarray (6)
I believe the answer is:
'old' is the definition.
(similar in meaning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for senior that I've seen before include "Elder (of two)" , "Higher in status" , "More advanced in years or authority" , "Higher in rank or age" , "Older or higher in standing" .)