Sort of strength in steel when tempered (7)
I believe the answer is:
tensile
'sort of strength' is the definition.
'in steel when tempered' is the wordplay.
'when tempered' is an anagram indicator (I've seen this in other clues).
'in'+'steel'='insteel'
'insteel' anagrammed gives 'TENSILE'.
(Other definitions for tensile that I've seen before include "Capable of being stretched, kind of strength" , "Able to be stretched" , "Relating to stress" , "Capable of stretching" , "Capable of being drawn out or stretched" .)