They have a decreasing effect on clubs! (5)
I believe the answer is:
irons
'clubs' is the definition.
(I know this)
'they have a decreasing effect on' is the wordplay.
I cannot really understand how this works, but
'on' is present in the answer.
The remaining letters 'irs' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for irons that I've seen before include "Fetters, handcuffs" , "in which one is restrained" , "instruments of restraint" , "Presses clothes" , "Prepares clothes" .)