Tablet, which is reportedly very strong (5)
I believe the answer is:
stele
'tablet' is the definition.
(term for a decorative stone or tablet)
'reportedly very strong' is the wordplay.
'reportedly' indicates a 'sounds like' (homophone) clue.
'very strong' becomes 'steel' (steel can figuratively mean strong).
'steel' is a homophone of 'STELE'.
'which is' is the link.
(Other definitions for stele that I've seen before include "Column" , "Ancient gravestone" , "Tombstone" , "Commemorative stone" , "Upright stone slab" .)