Protect from harm (6)
'protect from harm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for defend that I've seen before include "Fortify" , "Protect, fight back" , "Shield" , "Protect against challenge or attack" , "Stand up for" .)
(Other definitions for shield that I've seen before include "Protect - prize" , "Plateau" , "Piece of armour carried on arm to protect body" , "Piece of plated armour" , "Rampart" .)