Press joins small clubs (5)
'clubs' is the definition.
(iron is a kind of club)
'press joins small' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'joins' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
(Other definitions for irons that I've seen before include "English actor", "Jeremy ---, Br. actor; golf clubs", "for old convict to wear", "Metal leg shackles", "cuffs".)