One chief, on ecstasy, in pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'one chief on ecstasy' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'chief' becomes 'ch' (abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'ecstasy' becomes 'E' (informal term for the drug).
'a'+'ch'+'e'='ACHE'
'in' is the link.
(Other definitions for ache that I've seen before include "Persistent dull pain" , "Continuous hurt" , "suffering" , "anguish" , "Play up" .)