Chlorine not to be used? That’s a blow (5)
I believe the answer is:
clout
'a blow' is the definition.
(I've seen this before)
'chlorine not to be used?' is the wordplay.
'chlorine' becomes 'Cl' (chemical symbol).
'not to be used?' becomes 'out' (I can't explain this - if you can you should believe this answer much more).
'cl'+'out'='CLOUT'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for clout that I've seen before include "Cast not one before May is out" , "Influence; heavy blow" , "hit hard" , "Bash" , "Sock" .)