Long article about explosive (4)
I believe the answer is:
ache
'long' is the definition.
('ache' can be a synonym of 'long')
'article about explosive' is the wordplay.
'article' becomes 'a' (indefinite article).
'about' becomes 'c' (abbreviation for 'circa'**).
'explosive' becomes 'he' (abbreviation for high explosive).
'a'+'c'+'he'='ACHE'
(Other definitions for ache that I've seen before include "Persistent dull pain" , "Play up" , "Long ? pain" , "anguish" , "Dull steady pain" .)