Losing head, rants for a long time (4)
I believe the answer is:
ages
'a long time' is the definition.
(I've seen this before)
'losing head rants' is the wordplay.
'losing head' means to remove the first letter.
'rants' becomes 'rages' (I've seen this in another clue).
'rages' with its initial letter taken away is 'AGES'.
'for' acts as a link.
(Other definitions for ages that I've seen before include "Grows old like sage" , "(What seems) a very long time" , "Matures" , "... of Man, according to WB Yeats" , "Aeons" .)