At heart, 'Just William' is sturdy stuff (5)
I believe the answer is:
twill
'sturdy stuff' is the definition.
(type of fabric)
'at heart just william is' is the wordplay.
'at heart' says the answer is hidden in the clue (letters hidden in the heart of the clue).
'TWILL' can be found hidden inside 'just william is'.
(Other definitions for twill that I've seen before include "Strong woven fabric" , "Fabric woven to produce a surface of diagonal parallel ridges" , "Diagonal-weave fabric" , "Cloth with diagonal ribs" , "Material" .)