Stopping to put additive in shell (7)
I believe the answer is:
ceasing
'stopping' is the definition.
('cease' can be a synonym of 'stop')
'put additive in shell' is the wordplay.
'put additive' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'shell' becomes 'casing' (synonyms).
'e' going within 'casing' is 'CEASING'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for ceasing that I've seen before include "Stopping" , "Discontinuing" .)