Stop, relax and come down (8)
I believe the answer is:
restrain
'stop' is the definition.
(I've seen this before)
'relax and come down' is the wordplay.
'relax' becomes 'rest' (I've seen this before).
'and' means one lot of letters go next to another.
'come down' becomes 'rain' (raining is a kind of coming down).
'rest'+'rain'='RESTRAIN'
(Other definitions for restrain that I've seen before include "use handcuffs?" , "bar" , "Prevent (from excess)" , "Keep under control" , "bottle up" .)