Wise men heading off for a long time (4)
I believe the answer is:
ages
'long time' is the definition.
(I've seen this in another clue)
'wise men heading off' is the wordplay.
'wise men' becomes 'sages' (I've seen this in another clue).
'heading off' means to remove the first letter.
'sages' with its initial letter taken off is 'AGES'.
'for a' acts as a link.
(Other definitions for ages that I've seen before include "Matures" , "Long periods" , "Awfully long time" , "Yonks" , "Rock of - - (hymn)" .)