Spasmodic attack almost complete (6)
I believe the answer is:
fitful
'spasmodic' is the definition.
('fitful' can be a synonym of 'spasmodic')
'attack almost complete' is the wordplay.
'attack' becomes 'fit' (fit is a kind of attack**).
'almost' means to remove the last letter (most of the word but not all of it).
'complete' becomes 'full' ('full' can be a synonym of 'complete').
'full' with its last letter taken away is 'ful'.
'fit'+'ful'='FITFUL'
(Other definitions for fitful that I've seen before include "Spasmodic, intermittent" , "Occurring in short periods, not regularly" , "Intermittently stopping and starting" , "Restless or sporadic" , "Restless or irregular" .)