Component parts assembled by steel men (8)
I believe the answer is:
elements
'component parts' is the definition.
(I've seen this before)
'assembled by steel men' is the wordplay.
'assembled' indicates an anagram.
'by' means one lot of letters go next to another.
'steel'+'men' is 'steelmen'.
'steelmen' with letters rearranged gives 'ELEMENTS'.
(Other definitions for elements that I've seen before include "He and I are two" , "Basic parts" , "Parts of something or the weather" , "Components or weather conditions" , "Essential parts or the weather" .)