Smart alec knight, initially with no form of defence (4-3)
I believe the answer is:
know-all
'smart alec' is the definition.
(I've seen this before)
'knight initially with no form of defence' is the wordplay.
'initially' says to take the initial letters.
'with' means one lot of letters go next to another.
'form of' indicates an anagram.
'defence' becomes 'wall' (wall is a kind of defence**).
The initial letter of 'knight' is 'k'.
'k' after 'no' is 'nok'.
'nok' with letters rearranged gives 'kno'.
'kno'+'wall'='KNOW-ALL'
(Other definitions for know-all that I've seen before include "Self-styled polymath" , "Person who behaves like encyclopedia 94-3)" , "Clever clogs!" , "Pretender to great learning" , "Pedantic" .)