Calm down a striker beginning to explode (5)
I believe the answer is:
abate
'calm down' is the definition.
(I've seen this in another clue)
'a striker beginning to explode' is the wordplay.
'striker' becomes 'bat' (I've seen this before).
'beginning to' indicates taking the first letters.
The first letter of 'explode' is 'e'.
'a'+'bat'+'e'='ABATE'
(Other definitions for abate that I've seen before include "Subside like storm" , "Wane" , "Relax" , "drop off" , "retreat" .)