Mysterious character in ecstasy after work (4)
I believe the answer is:
rune
'mysterious character' is the definition.
(I've seen this in another clue)
'ecstasy after work' is the wordplay.
'ecstasy' becomes 'E' (informal term for the drug).
'after' says to put letters next to each other.
'work' becomes 'run' (both can mean to operate something).
'e' put after 'run' is 'RUNE'.
'in' is the link.
(Other definitions for rune that I've seen before include "Ancient character" , "Spell" , "Mystical symbol" , "Letter of an ancient Germanic alphabet" , "Old alphabet letter" .)