Unlimited iron props assembled underground (5)
I believe the answer is:
metro
'underground' is the definition.
(I know that underground can be written as metro)
'unlimited iron props assembled' is the wordplay.
I cannot quite understand how this works, but
'assembled' could be 'met' ('meet' can be a synonym of 'assemble') and 'met' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for metro that I've seen before include "The underground railway system in Paris" , "Parisian underground railway" , "Parisian underground system" , "Paris underground rail system" , "Paris subway" .)