The greatest Liberal seated in first class (3)
I believe the answer is:
ali
'the greatest' is the definition.
(film about Muhammad Ali)
'liberal seated in first class' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'seated in' means one lot of letters goes inside another.
'first class' becomes 'ai' (A1 - top quality).
'l' inserted inside 'ai' is 'ALI'.
(Other definitions for ali that I've seen before include "Fourth caliph of Islam" , "Lepidopterous floater, apian stinger, d. 2016" , "Mohammad . . ., . . . Hewson" , "Egyptian name" , "Person" .)