Release cog and coast (9)
I believe the answer is:
freewheel
'coast' is the definition.
(I know that coast is a is a more specific form of the action of freewheel)
'release cog' is the wordplay.
'release' becomes 'free' (synonyms).
'cog' becomes 'wheel' (I've seen this before).
'free'+'wheel'='FREEWHEEL'
'and' is the link.
(Other definitions for freewheel that I've seen before include "Coast (on a bicycle) without pedalling" , "Roll along in neutral gear, be unrestrained" , "Go downhill on a bicycle without pedalling" , "powerless to move" , "The cyclist may" .)