Surprised a defender (5)
I believe the answer is:
aback
'surprised' is the definition.
(I've seen this before)
'a defender' is the wordplay.
'defender' becomes 'back' (I've seen this before).
'a'+'back'='ABACK'
(Other definitions for aback that I've seen before include "impeded by headwind?" , "Sternwards" , "dismayed to be taken thus?" , "To take someone -- is to surprise them" , "(Taken) by surprise" .)