Press one, or raise indefinite number (4)
I believe the answer is:
iron
'press' is the definition.
(I know that press can be written as iron)
'one or raise indefinite number' is the wordplay.
'one' becomes 'i' (Roman numeral).
'raise' says the letters should be written in reverse.
'indefinite number' becomes 'n' (n can represent any number).
'or' written backwards gives 'ro'.
'i'+'ro'+'n'='IRON'
(Other definitions for iron that I've seen before include "medicinal preparation for the anaemic?" , "Wellington was known as the .... Duke" , "Club to smooth the cloth" , "Implacable - golf club" , "Heavy metal that rusts easily" .)