Long account by ambassador (4)
I believe the answer is:
ache
'long' is the definition.
('ache' can be a synonym of 'long')
'account by ambassador' is the wordplay.
'account' becomes 'ac'.
'by' is a charade indicator (letters next to each other).
'ambassador' becomes 'he' (His or Her Excellency).
'ac'+'he'='ACHE'
(Other definitions for ache that I've seen before include "strong feeling" , "Continuous hurt" , "Take an aspirin for it" , "Dull throbbing pain" , "Hanker" .)