Different in steel section (4)
I believe the answer is:
else
'different' is the definition.
(synonyms)
'in steel section' is the wordplay.
'in' says the answer is hidden in the clue.
'ELSE' is hidden within 'steel section'.
(Other definitions for else that I've seen before include "Otherwise; different from" , "In addition; different" , "In addition, besides" , "Besides; different" , "Eels (anag)" .)