Fits spring on messaging service (6)
I believe the answer is:
spasms
'fits' is the definition.
(spasm can mean a fit)
'spring on messaging service' is the wordplay.
'spring' becomes 'spa' (spa can mean a mineral spring).
'on' is a charade indicator (letters next to each other).
'messaging service' becomes 'SMS' (Short Message Service, text messaging).
'spa'+'sms'='SPASMS'
(Other definitions for spasms that I've seen before include "Tics" , "Cramps or contractions" , "Sudden convulsions" , "sudden bursts of activity" , "Muscular problems" .)