Martin possibly confused Brian, the fool (9)
I believe the answer is:
birdbrain
'the fool' is the definition.
(I've seen this before)
'martin possibly confused brian' is the wordplay.
'martin possibly' becomes 'bird' (martin is a kind of bird).
'confused' indicates an anagram.
'brian' is an anagram of 'brain'.
'bird'+'brain'='BIRDBRAIN'
(Other definitions for birdbrain that I've seen before include "Stupid" , "Twittering twit?" , "Silly person (with sparrow's mind?)" , "Fool" .)