Senior retired Liberal in fashion again (5)
I believe the answer is:
older
'senior' is the definition.
('older' can be a synonym of 'senior')
'retired liberal in fashion again' is the wordplay.
'retired' shows that the letters should be reversed in order.
'liberal' becomes 'l' (abbreviation).
'in' is an insertion indicator.
'fashion again' becomes 'redo' (I've seen this before).
'l' put inside 'redo' is 'redlo'.
'redlo' back-to-front is 'OLDER'.
(Other definitions for older that I've seen before include "More advanced in point of time" , "Increasingly mature" , "More senior" , "More ancient" , "Elder" .)