Ian Dury is often thought, at first, a Blockhead (5)
I believe the answer is:
idiot
'a blockhead' is the definition.
(I've seen this before)
'ian dury is often thought at first' is the wordplay.
'at first' suggests taking the first letters.
The first letters of 'ian dury is often thought' is 'IDIOT'.
(Other definitions for idiot that I've seen before include "Ass, ninny" , "one needing head examined?" , "Unwise person" , "Jackass" , "eg Dostoevski's Myshkin" .)