Piece of steel is somewhat flexible (7)
I believe the answer is:
lissome
'flexible' is the definition.
(thesaurus)
'piece of steel is somewhat' is the wordplay.
'piece of' indicates the answer is hidden within the clue.
'LISSOME' can be found hidden inside 'steel is somewhat'.
(Other definitions for lissome that I've seen before include "Oil mess (anag)" , "Slim, supple, graceful" , "Willowy" , "flexible" , "pliable" .)