Assembly of steel bridges very sophisticated (6)
I believe the answer is:
svelte
'sophisticated' is the definition.
(similar in meaning)
'assembly of steel bridges very' is the wordplay.
'assembly of' is an anagram indicator.
'bridges' is an insertion indicator (some letters go around or bridge others).
'very' becomes 'v' (common abbreviation).
'steel' anagrammed gives 'selte'.
'selte' enclosing 'v' is 'SVELTE'.
(Other definitions for svelte that I've seen before include "Elegant and of slender build" , "Slim and stylish" , "Polished" , "Slim and elegant" , "Slender - refined" .)