Foot not party leader for a very long time (4)
I believe the answer is:
aeon
'a very long time' is the definition.
(an aeon or eon is a long time)
'foot not party leader' is the wordplay.
'foot' becomes 'paeon' (type of metrical foot in poetry).
'not' suggests deleting specific letters.
'party leader' becomes 'p' (leader of the word 'party').
'paeon' with 'p' taken away is 'AEON'.
'for' is the link.
(Other definitions for aeon that I've seen before include "An immense period of time" , "Vast time period" , "Eternity" , "Very long and indefinite period of time" , "With lots of time" .)