Steel structure only demolished under pressure (5)
I believe the answer is:
pylon
'steel structure' is the definition.
(I've seen this before)
'only demolished under pressure' is the wordplay.
'demolished' is an anagram indicator.
'under' says to put letters next to each other.
'pressure' becomes 'p' (abbreviation).
'only' anagrammed gives 'ylon'.
'ylon' put after 'p' is 'PYLON'.
(Other definitions for pylon that I've seen before include "Structure supporting power cables" , "Tall cable carrier" , "Power cable supporter" , "High-tension power line support" , "Tower for carrying power line" .)