Demons forming one minute afterthought (4)
I believe the answer is:
imps
'demons' is the definition.
(I've seen this before)
'one minute afterthought' is the wordplay.
'one' becomes 'i' (Roman numeral).
'minute' becomes 'm' (abbreviation).
'afterthought' becomes 'ps' (ps stands for postscript).
'i'+'m'+'ps'='IMPS'
'forming' acts as a link.
(Other definitions for imps that I've seen before include "Mischievous fairies" , "Demons" , "Naughty sprites" , "Children causing trouble" , "Mischievous spirits" .)