Friend with strange ideas for defensive structure (8)
I believe the answer is:
palisade
'defensive structure' is the definition.
(palisade is a kind of defensive structure)
'friend with strange ideas' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend**).
'with' says to put letters next to each other.
'strange' indicates anagramming the letters.
'ideas' is an anagram of 'isade'.
'pal'+'isade'='PALISADE'
'for' acts as a link.
(Other definitions for palisade that I've seen before include "defence work" , "fencing" , "Fence consisting of stakes" , "Fence of wooden stakes" , "Enclosure of stakes" .)