In steel, perhaps, but capable of being stretched (7)
I believe the answer is:
tensile
'capable of being stretched' is the definition.
(I've seen this before)
'in steel perhaps' is the wordplay.
'perhaps' indicates an anagram.
'in'+'steel'='insteel'
'insteel' anagrammed gives 'TENSILE'.
'but' is the link.
(Other definitions for tensile that I've seen before include "Capable of being stretched, kind of strength" , "Capable of stretching" , "Relating to stress" , "Able to be stretched" , "Relating to tension" .)